Make it so that we can create multiple action bar set ups where it *memorizes* the set up, and can be saved.
So for example, I could set up a Debilitation page for my Rogue, where they take a certain array of memorized spells, and their action bar placement would be saved as well. Then I could open up a new preset, and set up a stealth focused-assassination page, in which I mix in other abilities, and change the overall alignment of the build to be more tuned to a different goal. For example, I typically have a different build depending on the fight I'm expecting. Against a boss, I'm likely going to take a build that will allow me to do high damage, with reasonably high survivability. Against a bunch of 'weak' enemies, I'll typically take a build more for repositioning and assassinating a specific foe or dealing heavy damage before the fight begins in earnest.
Obviously I can just make edits on the fly with the build setup, but to be honest, out of my approximately 70 hour or so playthrough, 2-3 hours was setting up and changing my build every so often to deal with new foes or whatnot. It's more a convenience thing, quality of life. If it came down to the wire, I'd rather have more content added into the game, than this, by far.
The action bar itself is non-negotiable, definitely needs to be made so that it doesn't randomly keep adding items to my bar. Abilities randomly vanish from the bar occasionally as well.
More atrocious than the action bar or skills, and is truly the critical thing, is that there needs to be a better inventory management system put in place. It's really frustrating to try and clear out all the items I have, or where to put them. The crafting system is non-intuitive, I barely did anything with it. Why would I craft a set of grey plate armor, when I can go buy a comparable set that has a ton of benefits with it. As an Archer the crafting arrows and whatnot is interesting, however the class itself is kinda boring. It would be greatly improved if we could 'enchant' our pieces using those souls and essences that we picked up, and maybe get some exceedingly powerful items if we dump enough of those items together. Where a normal longsword at my level might deal 100 damage, combining 10 of the best of each essence might give it a substantial power boost in the form of unlocking elemental damage of each type, in addition to say unlocking a few unique skills.
Speaking of skills, the Screaming Mandrake plant (I revisited my last save after beating the game) was pretty cool, we NEED more like that. Where you can find unique and interesting skills through decisions in game. I wish there was more a focus on attaining power through means like that. Venture forth into the deep dark vault of a Lich, for him to offer you the power of Undeath itself, granting you a unique skill, talent, or similar effect. The Spider's Kiss might grant you the ability to summon spiders now, access other new skills, or even enter a new location exclusive for those who accepted (like an area full of terrifying spider merchants, with spells and new quests to do).
I loved the game, don't get me wrong, I'm well over 110 hours of playtime total at this point (including pre-release, and 70 hours AFTER release :^) ), I'd just like to see even more content.