A lot of people already stated the obvious visual improvements, so I won't bother repeating those.
Instead, some game changing advice, which would switch the overall feel of the game.
• 1st of all, avoid cutscenes as much as possible. For example, 1 situation really sticks out for me every time I think about this. When hunting the royal wyvern for the contract, you climb trough a cave, to arrive to the top of the nest on the hill, where you fight 2 regular wyverns (if I remember correctly), after whose defeat, a royal wyvern flies up from nowhere, to surprise you. EXCEPT, it's no surprise, because the game played a cutscene to show me that, instead of just letting me experience this in the moment. Pointless cutscenes should be avoided in every game, it's so much more effective to experience events real time in-game. That makes it an experience that just happened to you, instead of taking away the control from the player, which is, imo, the worst thing to do, and a lazy way out, not to have to implement those events as core game play mechanics. When you avoid cutscenes, everything is more connected. I'm okay with the dialogue cutscenes, for the most part.
• Next on the chopping block, monster encounters. The monsters themselves are really cool, but also way too passive, which in most cases, removes the "monster" aspect away. They hit the nail on the head with some monsters in B&W, most notably the Bruxa/Alp. Super aggressive, leaves little room for you to breathe, dodges your attacks unless you stun them in some way 1st. Really justifies the description in the bestiary that very few witchers dare take contracts on these monsters. They ARE dangerous. If i see the lady in the cloak waiting for me behind some trees in the woods, I'm either turning around, or taking the long way around. (unless of course, you don't play on Death March, and have mods that increase the combat difficulty of the game. But then again, you wouldn't be the type of person to make these complaints then to begin with, would you?).
The opposite example of that would be the Leshen, which in MH:W was a pain in the ass to kill, and in TW3, you just spam igni until it drops dead. This ancient powerful creature is struggling to dish out an attack more than once every 10 sec. and when it does, it's so slow, even a peasant could avoid it, in which case, why do we need Witchers?
Needless to say, underwhelming.
Some mods that I use, hide this fact, and make the combat more exciting.
Random encounters reworked for example. Great idea in all aspects, but one. It's random. You have a good amount of customization for what you want, but the spawns are still random. I'll be fighting some kikimores, when all of the sudden, the mod spawns 3 ekkimaras and then a minute later, some ghouls. That makes for some exciting and fresh encounters, but the problem with it being random, still remains. Imagine if instead of it being random, it depended on the area of the world I was in, for example imagine that I was fighting a group of werewolves (simply because in the vanilla game, 1 werewolf is ezpz) and it so turns out, beneath us is a cave, which has a nest of kikimores we disturbed with out ruckus! Okay, they join the fight, which then gives me 2 possibilities; if the game was designed where that encounter would be deadly, I call roach, and either run away, or separate the threats... or if it's somehow manageable to fight both of these at once, or they start fighting amongst themesleves, giving me opportunity to strike some of them down... that then attracts some necrophages, because of the corpses that are fresh on the field. But not random ones, if I'm near a body of water, drowners swim out, if there's a cemetery nearby, ghouls show up.
So we deal with all of those like a good monster hunter we are, and decide to head back to town... Except, some monsters are also hunters, who should, if the opportunity presents itself, ambush their prey. So on our way back to town, while riding roach, we go trough a forest region, with the werewolf trophy hanging by the saddle. The smell attracts some predator from the area. It could be... another pack of werewolves, some bigger monster lurking in the shadow of the night, doesn't matter. The game speaks so much how you should stick to the beaten path, and then I casually gallop on roach throughout every forest, looking at the full moon, without a care in the world.
• Other then that, not much else needs improving, honestly. Some QOL improvements for sure. Like the mods which remove the ridiculous screen effects, like dirty/rainy camera lens. Are we following the Witcher with our camcorder? Is that what you're trying to portray? It's just silly.
Mods that remove the white trails from attacks/swords/arrows. Mods that remove the white paint in places where you're suppose to climb. Sure, just remove all of my sense of wonder for exploration, paint some red arrows while you're at it. I know that sounds mean to whoever came up with that idea, but the idea IS dumb, so...
Mods that add animations to some actions in the game (drinking, eating), not a big deal for me, but I know a lot of people complain about it.
I would much rather see more animations and interactions between themselves on just the regular NPCs we pass by, who by the way, should not repeat the same voice line, 20 times in a row, if I stand too close to them for a hot minute.
Aaand, that's about it!