I think a mix of both fixed and random could work well.
Random gear being more generic stuff (Like BG3's regular "+1" and "+2" gear with no fancy unique effects) that you can use to fill out your slots while you run around bumping into the fixed stuff that has more bonuses for specific builds.
Also having fixed gear having multiple potential spawn locations would be interesting. Meaning, they always exist within a run. But their placement can differ between runs. To mitigate the BG3 aspect of "Just running directly to the same places at the start of every run/transition into a new area to go pick up all the notable loot" thing and make it more of a natural progression of acquiring items when going through and clearing areas.
Outside of that, something to consider would be some sort of crafting system where you could build your own items, utilising effects you've previously found. Meaning you can make items you found that boost your build in Act 1 into having stats that are relevant in Act 3 instead of having to ditch the bonus that fits your build perfectly as soon as you progress into the game... This could also help alleviate the issue of shared loot, if you can craft a copy of singularly good items to allow use on multiple characters.
I think the best way to handle surfaces is to put them onto specific skills.
One of the main issues with surfaces across Larian games, is that every fight always becomes dominated by them because of how every single ability would be creating/interacting with them. Which made them kind of annoying and tedious, or outright frustrating (Trying to use things like Poison or Acid surfaces... But then any attack against a target in them makes them drop a blood surface under them that replaces it... To say nothing about how dropping blood surfaces on hit lead to DOS2's "Everything is an Electrified Blood surface" nonsense)
If players have to intentionally go after making surfaces, with the exception being very specific encounters that might have enemies based around producing them, as well as more intentional means of interacting with them (I.e. You have to actually target the water/blood surface to electrify it rather than it being a byproduct of hitting a target) you then reduce the relentless spam of surfaces everywhere.
Once this is established, there can be a lot more development into surfaces themselves. Having more combos, interactions and layers (DOS2's "Clouds" that can sit above a surface).
Which could lead to more build variety as you could spec a character into surface manipulation as a primary means of combat, while people who don't enjoy surfaces can stick to regular skills and not be throwing out surfaces constantly.
For me, the main thing I want, is more emphasis on LIGHT.
I found it really cool that you can light up braziers and torches dotted around the place by interacting with them or hitting them with fire spells. The lighting systems also made things look really neat.
It's just that such mechanics were ultimately pointless as everywhere was well lit (Even more so if you were one of the 90% of characters that had Darkvision in BG3...). Like, even in Shadow-Cursed Lands where this was more of an actual mechanic it was so underutilised because you got the curse immunity like 2 seconds into the zone (As well as those shadows that turned off lights in an aura whenever it would be relevant after that).
I'd love to see more actual dark places, so you have to properly think about light sources. Carrying torches, using fire surfaces, carrying around candles (Which I did in BG3, not for light but as a means to burn away webs/entangling roots), setting characters on fire, interacting with braziers etc.
Honestly, it's hard to really talk about this without knowing how they plan on doing skills.
Whether they want to continue Original Sin's classless system, or want to take on a more restrictive class based system.
Based on DOS2, one of the main things that I found was that there really needed to be a LOT more abilities to actually acquire. Especially when it came to mono-skill builds, like it always sucked if I wanted to have a character be a tanky tank person and there was like 2 abilities worth using from Warfare so you had to go Polymorph (For again, like 2 more skills worth using).
Really, what I'd love to see is an updated AP system from DOS. Instead of 5 AP maximum and getting 2 (3 with Lone Wolf) per turn and almost everything using 2 AP. Having a wider scope of flexibility would be cool. Like say, 15 AP maximum and getting 5 (8 with Lone Wolf) AP per turn and a plethora of skills that cost 1, 2, 3, 4 and 5 AP allowing you to mix up AP usages based on what's available (I.e. Cooldowns) or what the situation requires.
Also, would love to see movement be divested from AP. At least having a baseline movement capacity independent of AP, with optional "Sprint" style consumption of AP for further movement (As well as an AP sink for disengaging from targets). So there's more freedom to use positioning, as opposed to Original Sin where you just sat there like turrets because spending AP on attacking > spending AP on moving (Not to mention how this also neutered enemies, who'd waste all their AP running at you instead of attacking)
