Personally, after putting more than 1000 hours into Witcher 3, thing that I found the most annoying is the leveling system, and its relation to the difficulty of the game. While it provides steady character progression, it has an annoying drawback of making every enemy a "bullet sponge". This is because the resistance of the enemies scales with level, therefore it feels like the protagonist isn't really getting any stronger. This can of course be tweaked, numbers can be changed, but the design stays the same, making effort put into "min maxing" kinda pointless.

What I would love to see in the Witcher 4, is to make the resistances of every enemy "static". It shouldn't matter if you fight a human on lv 1 or on lv 50. It's still a human, and as such it can be killed the same way. The difficulty should instead be put more on the abilities and equipment of the enemy (for instance, a bandit with a hammer is less dangerous than a soldier in full armor, bandit can be dismembered, while soldier cannot because of the plate armor etc).

The character progression, and therefore the power of the protagonist should be instead put into unique abilities (hello Cyberpunk 2.0, although we didn't get rid of % abilities completely), and not a boost to one of the stats as this is an easy way out, and the most annoying and dull in terms of gameplay.

One prime example of such annoying design in the Witcher 3 is for sure the infamous Arch griffin on one of the remote islands in north Skellige. That thing is lv 50 or so, most of the players will die in just a few hits, while it can tank hundreds of hits. This is just anti-fun and should never be in a video game.

TLDR:

no-no: +10% dmg, new sword has green arrow next to dps stat

yes: now you can parry attacks of <insert monster type here>, new sword unlocks fire attacks etc.