1. A More In-Depth Combat System With Combos & Enemy Variation
Combat in The Witcher 3 is flashy and fun, but not really very diverse. You can try to get creative, but all you really have to do is master the dodge roll.
Quen, slash, slash, dodge roll, Quen. Repeat enough times, you win. Limited variety is afforded by the various flying monsters, and even that is mostly limited to a timed crossbow shot or Aard, followed by the usual slash-dodge pattern.
This really doesn’t do the Witcher universe justice. Kaer Morhen is the home of ultimate swordsmen, not the Dodge Roll Shaolin. Much more fitting would be a system akin to a fighting game: chained combos, directional parries, feigning, and spells that can be followed up with sword strikes.
A deeper combat system will require more challenging opponents. In Wild Hunt, most of the humanoid opponents are way slower than Geralt, which leads to them having to compensate with numbers. Monsters, on the other hand, are basically made up of patterned behaviour, which you figure out and then counter with an appropriate pattern of your own.
It would be interesting to see more small-scale battles with opponents that can match the witcher with skill, not size, numbers or scripted random teleporting. Being pitted against an enemy that knows the same moves you do can lead to memorable, jedi-style duels.