First of all: Divinity Original Sin is already a great game and one of the best games I've played in the last years. So don't get me wrong, this topic is not about "complaining" or "whining", not at all. It's my feedback after playing the game for more than 40 hours in beta and for more than 70 hours after release. It's sole purpose is to give Larian input what could be improved in the game to make it even better. Why not striving for perfection?

I will tackle design issues, smaller and bigger fixes, usability improvements and polishing.
Let's start:
1) New and improved journal for better orientation/questingI don't know if many of you have played Inquisitor. It's an old-school RPG which combines (bad) Diablo-like combat with traditional quest-driven open storytelling. The game has huge amounts of text, many dialogues and it is very similar in terms of "strictly no hand-holding" in terms of quests and orientation. But Inquisitor has in that respect one important element: on top of the normal journal with only the brief quest description it also features a kind of "personal diary" in which the lead character notes every hint he could get for solving his quests. (* I've put a screenshot of this second journal at the end of this post so that you can follow me.), e.g. when he finds certain quest relevant items or when he talks with people who have information about locations or other people or whatever.
I would really recommend Larian to look into this game and think about a similar second journal for D:OS. Right now the game not only offers little to none hand-holding, it also refuses to give proper information in the journal. If you read a note for example, the information won't be available at a central place to look it up, no, you have to search the note in your inventory again. Dialogues with NPCs won't be stored at all (at least it's deleted when you leave the game). It would be a great tool to have a second diary in which stuff like dialogues with NPCs and animals, notes/books you've read, information you've gathered on the fly or items you've found. Forcing the player to go searching for their own stuff is a design decision and I have no problems with it. But then you should give players proper information and not force them to deal with a clunky inventory and with writing down things of their own (without knowing whether they are important or not). Hardcore old-school is cool, but it should be fair and there are obviously solutions to improve the system in place imo.
2) Hotkey for the hotbar and more hotbar space
Well, this is kind of obvious and was demanded several times now. I don't know who at Larian thought that clicking on a really, really small icon at least once in almost every turn in combat was a good idea. Of course browsing through your hotbars need a hotkey to improve usability. On top of that 30 spots for spells, skills and items are not enough for late-game characters. Ranger for example can use 23 different arrows alone. Now add some skills and many some spells and items on top of that and well, the space is filled. So please give us more than only three hotbars (five should imo be enough) and made them browable with an hotkey.
3) Add "repair all" and "identify all" for the whole party
That's again a usability issue. Right now identifying and repairing stuff is just tedious because you have to click on each equipped item in order to repair it. That's a lot of unnecessary and NOT fun clicking work which could be done with only one click instead. It's even worse if you want to repair stuff from other party members. In that case the only a little less tedious trick is to drag a repair hammer to your hotbar and then click on each equipped item of your party member (without that you have to actually deequip the items and send them to the other character, repair them, and send all back). So please, Larian, this stuff is not fun. And everything which is not fun shouldn't be in the game. I'm not talking about the system in general but about the clickfest coming with it. Just give us an option in the dropdown menu of the repair/identify tool to "repair all" and to "identify all" (which should apply on the items/gear of every linked party member).
4) Apply sneak command to every linked party member
Some problem as in #3: usability. It's kind of stupid that only the directly controlled player goes into sneaking mode if you are still linked to other party members. Sneaking while your other party members follow you makes of course absolutely no sense. You naturally delink your sneaky character before. If you want your whole party to sneak there is absolutely no reason to apply the sneak command on each character individually. So please, Larian, if I press sneak mode, put all linked party members in sneak mode. If I want to go sneaking alone, I delink the char first anyway.
5) Enable shared gold between linked charactersAnd another usability issue: bartering. Right now the game treats every character of your group individually, ignoring the fact that you as player have control over the whole group. So you for example can't buy a bunch of arrows for your ranger if only your melee has a lot of money but your archer none. You have to move money or items for or after a bartering operartion which is completely unnessary. Of course I know that this was made because of co-op. But think about that: I don't speak about the whole party. I speak about LINKED characters. Linked characters are always the players one player controls so there is no reason why these character should have their own gold stash. This is just a tedious mechanic which doesn't enhance the gameplay experience but makes it more clunky without any benefit.
6) Enable walking on the mapAnd...another usability issue or possiblity for improvement: add the option to move your party on the map. In this game you travel a lot and walking speed isn't exactly really high. It would be a nice feature for convenience to be able to control your party on the region map with a better oversight. That would be especially useful for areas you've already cleared of all enemies.
7) Add inventory sorting filter for "item type"Honestly, I don't understand that. Why making a filter for weight but not for item types? I can understand the "latest" filter and the "value" filter but weight? Please, Larian, add a filter that sorts the inventory according to item types (e.g. armour, weapons, spells/scrolls/skills, ingredients, consumables, arrows, miscelanous). The different tabs are ok but they don't help much for crafting. An ingredients tab is kind useless if you want to combine an ingredient with a weapon. Right now you have to do most crafting in the general tab of the inventory which can become really tedious if your inventory is full with dozens of different items. Right now, inventory navigation takes a lot of time that isn't much fun and I don't think that this is something that can't be improved.
8) Relocate crafting/blacksmithing notifications to the actual inventory screenWhen you try to craft or smith something and lack the proper skill in the inventory a message above the currently controlled char appears that their skill is too low. Unfortunately this note is rather badly placed since you do crafting in the inventory and it's tedious to move around the inventory all the time in order to be able to see your character and investigate whether they arbe able to craft a certain item or not. Please relocate the "skill lacking" note to the inventory at the exact place where you wanted to combine two items.
9) Display equipment comparison of each party member in barteringThis was again already mentioned multiple times by other users. If you want to buy new gear for party members the comparion functionality only compares shop items with the gear the currently controlled character wears. This is unintuitive and tedious and should be changed. Items in shops should be compared to the currently selected character in the bartering menu.
10) Make quest items unsaleable and undisposable and give them a special tabI've read a lot of complaints that people lost their quest relevant items (e.g. the reveal scroll for Evelyn's cave). That's very disappointing in every game and should be avoided by design. Even if you haven't lost the item it's sometimes very tedious to search a single item in an inventory which contains 100 items or more. So imo it would be a good ideda to add another inventory tab for "quest items". Every item in this tab shouldn't be saleable or disposable. You should only be able to send them from one character to another and you should be able to easily find these items in their own inventory tab without being forced to dig though your whole inventory (which is the opposite of fun imo).
11) Disable crafting if no further improvement is possible
Right now you can for example combine essences of the same element multiple times with the same armour and it always shows the crafting process bar and the essence disappears. But the armour can only be improved once with every essence. After that the essence is just waisted without and further improvement and the game pretends by showing the crafting process bar that the crafting process would be successful. Imo crafting shouldn't be possible if no further improvement is possible. Or give at least a note that it doesn't help anything. If not people have to find out "the hard way", being forced to reload the game or just waste the ingredient.
12) Add legendary/unique stuff to boss drops and treasure chestsThe whole loot system in the game is based on randomness. In general this is kind of cool because you never know what you get. The problem right now is that bosses or treasure chests don't offer really better items than random chests or merchants. That takes away from the satisfaction to defeat an enemy or find a treasure chest. There should be at least a (much) bigger chance to find good stuff there or Larian should just add unique items to certain containers/boss drops. Maybe they should think about a second pool of random items which only contains legendary and unique items that can only be dropped by bosses or found treasure chests while normal chests and merchants are filled by another pool of random items which contain only normal, magical and rare items.
13) Disable the random system for spells and ingredients at merchantsAs much as I like the random loot system I think it's a great disservice for certain items, namely spell- and skillbooks and ingredients. Right now it's incredibly difficult to find certain spell-/skillbooks at merchants for no apparent reason. In fact the system forces or motivates a lot of people to reload the game over and over until they get the skill-/spellbook they want to find. This is especially true for melee/ranger and rogue skillbooks which can't be crafted with scrolls. Some skills are literally gone later in the game and if you decide to respec your character you probably find out that some spells/skill are just not available anymore. This should be fixed. People should have a chance to get new skills and old skills without being forced to reload the game over and over. Same is true for ingredients (sinew is a famous example).
14) Fix the trigger for the attack on the Homestead in Phantom ForestRight now entering the Phantom Forest by clicking on one of the two bridges in the Luculla Forest triggers the attack on the homestead. While the homestead is under attack you can't find new rooms there which can even cause that you won't find NPCs like the secrets trader or the respec trader until very, very late in the game when you don't need them anymore. This is especially true if you triggered the attack "too early" without knowing that this leads to the Homestead being on hold (you don't know that the respec and the secret guys are there in your first playthrough) until you solve this very quest. Imo the script for the attack on the Homestead should be triggered when you enter the northern-eastern area of Luculla Forest and not when you enter Phantom Forest (which is counter-intuitive because you just entered a new area just to travel back to the old). And it probably shouldn't block the activation of new portals in the Homestead.
15) Disable rot infection in HiberheimThis is easily one of the most annoying things in the whole game. There is at least one item (a wand/staff) in Hiberheim which inflicts rot on every character that has the item in the inventory. The problem is that you can be infected quite early in Hiberheim without having found out about rot before (if you enter Hiberheim before being in Silverglen). That way you can play for many hours in Hiberheim, constantly suffering from rot without knowing how to cure it. But instead you are constantly forced to heal this character which is very tedious. So please disable rot infection from every possible item in Hiberheim.
16) Disable multiple pets (for kickstarter backers)This maybe only applies to kickstarter backers witha a personal pet. If you can't summon a permanent (which means non-vanishing) pet, just continue reading with point 17.
Right now every char in the party can summon the pet. At the same time. So you can summon four permanent animals, all looking the same, all four with the same name. I think the problem is quite obvious. It's not only incredibly OP but also very immersion-breaking. Please limit the personal pet to one at the same time. This is already powerful enough.
17) Add a dialogue option in Cyseal for information about the regionThat's again an issue for beginners or people who need/want a little be more guidance without using quest markers or stuff of that kind. But right now there is absolutely NONE information about areas of the game. I would recommend at least for the first area in Cyseal a dialogue option (maybe for Aureus who also gives you most of the regional quests) which tackles the environment. I think of something like that.
Char: "What could you tell me about the surroundings of Cyseal and the enemies there?"
Aureus: "My scouts have told me that the undead are especially strong in nubmers and dangerousness in the north and the east. In the west the situation should be better but I heard of some scattered groups of undead rambling through the countryside. On the beach in the south-west there is the ork army which is proably not as dangerous as the undead in the north and east but mor dangerous than the undead in the west."
Of course my writing skill is limited here but you should get the picture. Just enable the player to gather some information where to head out first without being forced to experience it "the hard way". That might be cool for the hardcores among us, but the complete lack of any guidance can be very frustrating for beginners and people not used to old-school games from 15 years ago.
18) Add sarongs made for non-intelligence based classes/charsI've played this game for more than 100 hours now but I haven't found a useful sarong yet that doesn't have at least an intelligence requirement of 6 or 7. There is basically no magical sarong for melees, rangers and rogues in the whole game. This is very strange since every character can wear a sarong. Every other gear spot offers items which can be useful for every class while the sarong spot only offers items which are in fact solely useful for wizards/priests (or every intelligence based class/char). That makes little sense to me and renders the gear spot (almost) completely useless for a fraction of possible chars.
19) Add endgame content to the game (lvl 20 bosses)I think the game is pretty linear. You can't find any enemy in Cyseal for example which has a level above 10, no enemy in Hiberheim above level 13 and no enemy in Luculla Forest above level 16. I wish there would be some super high-level enemies/bosses hidden everywhere in the world (maybe in small dungeons) which you can't fight (at least not without a realistic chance) until very late in the game. Just think for a second about Kangaxx, the undead super hard lich, in BG 2. This enemy was just hidden in a crypt in the middle of Athkatla. You could basically enter the crypt at a very low level just to be ripped to pieces. But that caused a certain effect: you remembered that special enemy and while following the story and leveling up you always though about your chances to go back and kick this stupid lich in the ass. There is a certain charm to find secrets and enemies in levels that you can't defeat early and for which you have to come back later in the game. In Divinty: Original Sin ther is absolutely no reason to return to Cyseal after you've entered Luculla Forest and that's pretty sad. Let's take for exaple the source abomination in the Black Cove which can only be found with Pontius's key for the big gate. Why not making this enemy level 18-20? Players would probably think "WTF? Are you freaking kidding me..." They would be defeated several times until they give up. But they will all swear to come back and take the final revenge later and find all the high-level loot hidden there. There is no such element in D:OS right now. End game content is exclusively limited to the north of Phantom Forest. That's kind of disappointing (and could be improved).
20) Make endgame content more difficult/balancedThere is a certain degree of diffuclty decline built in this game. Most fights above level 14-15 are way too easy because the player's party has become too powerful. Mages have superiour group control with almost sure disabling spells, rangers are jack-of-all-trades who can charm almost every enemy and melees are effective killers with immunites against every element and high damage output, especially when properly buffed. While you are kind of forced to use summon early in the game up until level 10-14 you barely use them afterwards because you just don't need to. I personally think that the game lacks some challenge in the end game which is caused by overpowered spells and skills, overpowered resistancies and too harmless enemies. Healing potions at the same time become completely useless once you've reached level 10-12 because they only heal a very small amount of health (in the end game they heal less health points that you lose in average by a single blow). You don't need any potions in combat anyway later in the game. You don't need any scrolls (apart from those which you use for spellbook crafting once). Don't get me wrong, it's still fun to play and not TOO easy. But it's definitely easier than the first 20 hours in the game, of course because you both know the systems and tactics better and because your party gets slightly overpowered with enemies being too harmless. I think the game would profit from at least more dangerous high-level enemies who put out at least doube the damage they do know on average (especially on level 16-20).
So, that was my top 20 right now. Feel free to discuss and share your own thoughts and experiences. But please do it in a civilized way. And sorry for the LONG post...

*Screenshot from the "personal diary" in Inquisitor