Me and a friend of mine have been playing couch co-op RPG's for quite a while. From Dungeons and Dragons Heroes  to the Champions series. From Buldurs Gate 2 to Diablo 3. We even turned XCOM and XCOM 2 into couch co-ops. But its Original Sin that is King so far, the turn-based tactical combat sees to that. With that said there are a few annoyances that I hope will be rectified in Original Sin 2.
These are my 3 suggestions:
1. At the moment we are unable to access the contents of rucksacks during a trade. If we wish to sell the contents of a rucksack we have to (as far as I'm aware) back out of the shop, open inventory, open rucksack and then manually hold and transfer all items we wish to sell from our rucksack into our inventory before we can sell them. This is an annoying system that will hopefully be rectified.
Hm, not a bad idea probably.
2. I think that when assigning items to the hot bar, they should stay assigned until manually removed. At the moment I have one character with all his grenades assigned and another with all his scrolls assigned but when I use my last scroll or grenade the slot automatically un-assigns the item from the slot. So when I replenish spells or grenades that were assigned but are now not, I have to keep re-assigning them back to the hot bar. I think it would be better if all items assigned to the hot bar stay assigned until manually removed. If you run out of an item that is assigned to the hot bar it should just dim grey to signal there is none left. Then when you pick up more, they will already be assigned to the hot bar ready to use. Again, nothing major, just a little tweak that would make our lives easier.
Being able to lock bars would have been golden in EE already. That whole controller development screwed mouse handling a lot and a lockable bar would prevent us from ending up with a skill icon stuck on the mouse cursor so many times.
(The bar not responding to mouse clicks at all, requiring several clicks, is another problem but the cause must also be the controller development, can't remember this happening in Classic.)
I'd consider it an ideal solution to have bars lockable but still modifyable by dragging something on it while e.g. Shift is pressed. I think that is how it worked in WoW ages ago (and probably still).
Remembering items in a slot even after a stack has run out is also a very good idea. Most want to keep items in the same slot the whole time.
(From what I've seen in videos though, the skill bar seems to have changed a lot in D:OS2.)
3. And finally, I'm sure others have mentioned this, but tracking quests. I like that the game makes you figure out where to go by yourself and not holding your hand but sometimes this can be a little irritating. Maybe when a quest is being tracked, a huge circle should appear on the map, showing you that your next objective is somewhere within that area. That way by  talking to random NPC's like some guy in a tavern who might have additional information on your quest could reduce the size of the circle when tracking a quest, narrowing down the area. At least this way you would always have a general idea of where to go.
Anyway these are my 3 suggestions. Let me know what you think! 
 
 Well, how much more quest help is needed ?
I've watched people ignore quest markers (the yellow ones) and the quest log, I've watched people skip exactly THAT dialog option that is quest relevant, I've watched them ignore books, quest scrolls and a lot more.
Do you really think that adding even more quest helpers can help ? People ignore what can be ignored.
Happens with quest text, happens with dialogs, happens with already placed markers, even happens with skill descriptions.
The very biggest problem Larian has is that a lot of players start playing this game thinking they know everything from other games already and even expect a lot of things to work like in those other games.
Original Sin is dialog and text heavy by design, EE had all information somewhere in the texts and did a much better job at presenting this information than classic had done. If people don't read in a read heavy game, then it becomes a problem, but the cause of the problem is not Larian, although they have to 'fix' it ...