There seems to be so far no topic about suggestions regarding a new game but now as Divinity 2 is available and there should be some experience around it might not be wrong to eventualy do a small "summary" and post what players would eventualy like to see in a future game.
(this is just "my" oppinion, so nothing set in stone, even if it might sound not always that way I thought the game was enjoyable and any "suggestion" is make is only a "suggestion"

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At first of course the simple things.
Its probably a somewhat to early for that as maybe not even the developers of Divinity have thought about that point so far who knows but I think it might be not wrong to say that the gameembryo engine is showing its limits and is starting to age. Particularly when you remember that its already in use long before Divinity 2 even before Oblivion (Morrowind?). It might be feasible to eventualy discuss a new approach for the future eventualy. Some that might even go in conection with a better gameplay (see below).
But I think if the next game is trying to be at least in some aspect similar with Divinity 2 it might not hurt to check and discuss other aspects and engines. For example with "flight units" (wee Dragons!) a engine like used in ARMA 2 might be interesting!
See Screenshot.
The engine from ARMA is originaly a engine for a military game but since it also includes air units and very big distances with acceptable visuals it might be interesting to check it. It would definely allow a limitless use of "Dragon Powers". For the case any next time would use this kind of gameplay again (it really had good potential, but it feelt with Divinity 2 always "short" cause of engine limitations).
The Unreal engine in its newest update offers as well visualy stuning qualities and is used with many games today and has great tools and many Unreal games have seen as well quite a lot of modifications. Though I cant say what the limits are but it was at least possible with the Unreal Engine 2~ to make quite a lot of "realism" (Red Orchestra, Darkest Hour etc.) which would even feature tank combat on long distances.I am sure there are more engines to think about but that are at the moment the ones on top of my head. If people have more ideas/suggestions regarding that point dont hesitate to share them!
Something I would like to see in a new game more are more skills and use of them particiularly for warriors which choose to use a sword and other weapons for close range. More variation of skills and attacks. I think there should be also attack skills for someome he can choose to specialice. A long two hand sword should eventualy give you the potential attack 2 or 3 enemies directly facing you while a shield could give you the option to block a attack and 2 swords might give you something the advantage and dissadvantage of both.
The same should as well count for mages and othe classes. A mage could have here a lot more in the range of protection and attacks or something to "attack" the protections of others. Since as a mage the most dangerous enemy might be another enemy with magic attacks (cause of the range) you should get eventualy spells which can block attacks from a certain "school" of spells a protection of water against fire for example. If the enemy is using such kind of protection you would have to remove it first with another spell and then attack him. For a warrior this could prove as well to be a good choice since the class system is a open one but one has to keep in mind that a warrior never will get "good" in it and has to sacrifice other skills if he wants to from everything a bit. If you really want a good protection against magic you have to specialice as mage. If you want a really good protection against physical attacks you should try specialice as fighter etc. Of course characters should get the chance to temporarly push their protection with scrolls? or Potions? Maybe with chanted armor?
The next game should in my eyes as well make a difference between magic, fire and cold attacks and offer protections against it. The next game should get a bit more tactical in general but still remain as a game with strong focus on action of course.
I think there should be particularly a lot more visible differences between the archetypes of classes a clear warrior should feel different to a mage or ranger. Particularly when you engage enemies. I would love to see a difference here. Maybe with some armor, or anything similar. There could be different armors with different weight and look. A chain mail for rangers or at least something from other quality. There should definetly be a way to tell a difference between someome who is specialiced in the use of bows or ranged attacks and magic powers.
The world though should definetly feel "more" realistic though. Not realistic in physics or anything. I mean more realistic in locations, people and NPCs. It is very important for a RPG that you can try to step in the world and "play" a role and feeling a need to play that role.
For example. It is hard for me to believe that some world actualy is really a "living" and "breathing" world when you have somewhere in the wilderness a single merchant, farmer, soldier or what ever right next to a horde of goblins or thiefs or other kind of hostile creatures that would attack "only" you.
So I would also like to see a "biger" world for the next game particularly when you have in the next game eventualy again a chance to play as a "dragon" or have a chance to "ride" one I really liked the idea in Divinity 2 but it just feelt way to short and not that well thoght out a lot of invisible walls and other limits you had to face. Even if that means that the world will only be set together in smaller regions it doesnt matter as long the conection is done in a inteligent and believable way with canyons, hills and lakes.
I would also like to see eventualy more ways to "shift" your apearance in a new game with spells eventualy. Maybe you could use such things for your adavantage. Use a spell to shift your appearance as a wolf, or other anaimal and trying to sneak around a enemy and his defences. The better the scroll the less likely it is that the enemy might detect you eventualy?
A next thing for the case that you get again a chance to play a dragon or change your size in a similar creature please give us players a possibility to "land" and "walk" and not always fly. It would be also good to have a chance to attack on the ground as Dragon usual NPCs could flee as soon they see you even! Would be quite funny. I really missed that. I know it would be a quite powerfull attack but it could be balanced in a way that you cant do it everywhere, like in dungeons and very small locations could prevent you from a change and attack. You had in the world anyway "Dragonhunters" they could have weapons for attacking which deal more damage.