Points comparing to previous TW games:
1- Less QTE. Really, I can live with them, not a big deal, but they usually don't add anything interesting to the game. If anything, the opposite.
2- Both games were too damn easy at every difficulty setting. Look at Dark Souls as your next reference, please. And actually, look at Dark Souls even for the combat system (but giving it a more "fast" approach).
3- Underused urban areas. Cities in both games were pretty to look at, but they felt empty, with tons of NPC wandering around just for cosmetic reasons and very few chances to actually interact with them. TW2 in this was even worse than the first one.
4- No more "floating" or abstract controls. A more accurate response and a more direct control over your character would be great. Something like Risen/GTA could be nice, giving the player the ability to jump, to climb everywhere it makes sense being able to, to crouch for stealth in any moment, and more generally speaking, to interact with the scenario in more ways. Oh, and put some proper stealth system in the game. Entering someone else's house I would expect to be at least watched carefully by the owner. And I would expect to make him angry if i steal.
It's not just "realistic". It also opens more chances to make different and interesting quests and reward the player to come out with brilliant ways to reach guarded treasures.
5- Better minigames or no minigames. I'm not against them as an absolute, but you should really try to make them fun, interesting and rewarding. Or ditch them entirely.
6- Immersion and continuity in exploration. I don't care if the story takes place in one single reign or twelve different ones, what I actually care about is how large, detailed and interesting is the area I can actually explore. It's pointless to scale up your game and claim "this time Geralt will travel for million of miles" when i can actually explore just three small areas like in TW2.
7- Related to previous point: don't put in the game eight cities filled with lifeless zombies, give me one filled with interesting characters that offers me a lot of interaction instead. Khorinos in Gothic 2 is a perfect example of this.
8- Don't go for random loot. I know it can be an easy way to fill the game, but special items found in unique circumstances are far more rewarding and interesting.
9- Monster tracking and hunting should be expanded by far. I mean, that's the very defining activity for Witchers. I want to be forced to study my targets' habits, learn how to lure them into the open, how to take them down.
And wait... related to this last point, I just had an inspiration.
Imagine The Witcher 3, quite similar to the previous two on the social/narrative side, maybe bigger and longer, of course, but you got the idea: with human towns, compelling NPCs, dialogues, etc... And now on top of tha add a combat system inspired by Dark Souls, which is at the same time fun and challenging, plus large wild areas where to hunt, lure and kill large and/or dangerous monsters in a Monster Hunter-like fashion, enriched with some puzzle element on top of it (see point 9).
Wouldn't it be the damn best game ever?