Things I wish CDPR improve in TW3:
1) As many posters already mentioned, the
UI should be separate for PCs and for consoles. UI must be better. TW2 has a horrible UI, which is very unfortunate, considering the game's overall brilliant content.
Because I dislike making judgement statements, I'll explain why the UI is bad in TW2. Firstly, what constitutes a good UI? According to the branch of computer science named HCI (human-computer interaction), it's the UI that minimizes the amount of time and clicks/taps/button-presses from the point the user wants to make an action to the point when this action actually happens. Let's make an intentionally obvious example. The CRPG game UI designer wants to create a sword swinging UI and he has in mind two possible ways to do it: one will require to press one button to raise the hand with the sword, pressing one of the 4 buttons to choose attack direction, and a mouse click to execute the attack; the second idea is to make a random attack in one mouse click. It isn't hard to guess that the users, in general, will prefer the second model to the first one because the first model is a chore to master, without adding much to the fun factor. I'll make a note here that if the game was all about fighting simulator and not about much else the first model might have been preferable to the second one but in RPG sword swinging is not the corner stone of the game. Let's discuss the actual examples of bad UI from the game.
The alchemy UI. To drink a potion you need to: press a ctrl button, move the mouse to the middle and click the LMB, move the mouse to the left and click drinking menu, if you don't have the potion you need you need to go back, move the mouse to click on the alchemy menu, make a potion by clicking numerous number of times, go back, click the drinking menu again, click and choose potions to drink, wait forever for the drinking animation to go, and... you'll get back to the menu again where you have to press back twice to get out of the meditation mode. This is A LOT of useless clicking and pressing, which made the process a click-press fest instead of fun fest. This is how I'd have made it for PC: press one button (i.e. 'm') to get into the meditation mode, which will open the screen, where you can brew potions, sleep/meditate, and drink all-in-one screen. Then after everything is set up and "done" is pressed, the interface disappears, Geralt would sleep/wait then drink whatever is selected, stands up and the game keeps going. That's a dozen less clicks and presses.
And the most disappointing about the UI in TW2, which was made for consoles exclusively, is that it's relatively cheap to make right comparing to even a single average complexity quest. I wish CDPR to design two UIs from the beginning: one for PCs, one for consoles.
2)
Return the alchemy from TW1, it was the best alchemy system I've ever seen, and I've seen many. Really, it had usefulness, length of effects, toxicity, and special effects related to them perfect, except the DeVrie's potion, which was useless because there were no invisible monsters in the game. As the sayingы go: "Best is an enemy of good", "Don't fix what's not broken"... well, that was the case in TW2. And UI for consoles didn't help either.
3)
More and better interaction with Geralt's friends, if they appear there at all considering the location of the next game. TW1 was better in this aspect than TW2, where your friends are very static right after they said their introduction lines. Let them comment on everything going on in the game, let them give you their personal opinions and/or help on many quests, except for some monster related, of course, where Geralt is an expert and should know what he's doing.
4) It would be great if there were
less useless junk. You cannot avoid collecting junk even if you don't want to because it's so tedious to pick
5)
Return the visuals (2D style) for dice poker from the TW1. And if CDPR wants to make it better they should make the winning condition as a bankruptcy of Geralt's opponent, which can be achieved by wise betting like in real poker.
6) Music in TW1 was better than in TW2.
I would like to have TW1 style of music in TW3.
7)
Fix the annoying repetitive monologues, which are triggered every time you approach the . They are good to listen for the first time but it gets very annoying when NPCs repeat their very short lines in a loop. Either make those loops longer of fire them one time and then let them say some less annoying general phrases like "how's it going?" or some ambient noise where you don't distinguish any voices.
8 )
I wish finishers back in TW3, but better than in TW1. There are no finishing moves in TW2, there are stupid finishers' cut scenes, which take you out of combat, you don't control anything and you loose orientation in the process. It would be awesome if there were finishers, which happens in-fight and your enemies won't wait for you too, therefore the finisher when you're fighting multiple enemies should be very swift and it's the last enemy around it can be something fancy. But the most importantly:
IT MUST BE IN-GAME, not in the cut scene.
Things I think shouldn't be in the game but I'm rather neutral about it:
1) I won't cry if junk crafting is gone from the game completely. IMHO, the witcher is not a generic trash collector. CDPR should save it for the Cyberpunk game where it is way more appropriate.

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2) Ridiculous rate of monster respawn. I don't know why did CDPR do that but it was unnecessary. If difficulty is required I'd prefer tougher monsters to respawn every 10 seconds.
3) Please, remove anachronisms from the game. From time to time Geralt and Co. shows knowledge of genetics, nuclear physics, theory of evolution and other too advanced scientific knowledge for the medieval world (yeah, I know that some of those were in the books too, but it shouldn't be there either).
4) I'm not a fan of monster trophies as a stat booster. That's silly. Bring back royal hunter from TW1.
Things I hope CDPR
won't do in TW3:
0)
REMOVE QTE from the game for good. It sucks, period.
1) I will be disappointed if Nilfgaard will be reduced to stupid Nazies-like everybody-is-evil empire, glimpses of which we saw in TW2. Give them rational reasoning, make them understandable in some ways. I'd prefer them to be pompous hypocritical state that "spreads the culture to savages" than cartoonish evil.
2) I will be disappointed if the Wild Hunt will end up as a absolute evil elves from other dimension, who want to conquer the the Witcher's world... That's a very boring plot, it would be good if it were something more complicated than that.
3) I would be disappointed if the plot of TW3 will be about "saving the world" either from Nilfgaard or from the Wild Hunt. Such cookie cutter plots are already in every RPG, so keep it original. What was strikingly better about TW1 is that it was not that type of scenario. Hopefully, it won't be also about "saving the princess" type scenario, which is also beaten to death. As a suggestion, what could be an interesting and not too many times used plot is sneaking out of Nilfgaard with Yennefer where you get in all sorts of circumstances and troubles because of the war that is going on.
4) I would be disappointed if TW3 will be empty open world game where is nothing to do except trash hunting like Fallout3, Skyrim and other Bethesda's lazy crap. Making open world with good plot is practically not feasible unless somebody is willing to make a game for more than a decade. So, I wish CDPR to stick to their guns what they do the best and are the best in: non-linear plot driven RPG with restricted freedom of travel.