Better AI (clarification below)
I don't know what other players would think of this, but there were some parts, especially in White March, where kobolds, for example, would try to kill you with bows from really, really far away and somebody else would be blocking your part, or the kobolds would be unreachable, unless you cross quite the space to get to them.
My point is, I really liked this real monster-like behavior. Kobolds should be dumb, but they should probably be smart enough to try to do hit-and-run attacks, for example. So, I really realize it's going to be a lot of work, and the game would become a lot harder than it is now, but if Obsidian could maybe implement this kind of AI behavior, it would be awesome.
Another example would be when enemies would charge you, and you would position your tank around a door, blocking their path, while killing them with your range heroes or mages. This always kind of felt like I was kind of exploiting a weak part of the system... If my tank were to block the door and the enemy was smart enough, of course they would try to remove the tank, but maybe they will also switch to bows / crossbows and target more range heroes / mages instead...? I don't know. There could be a lot of options here... But that would also kind of justify why I would want a plate mail on my mage, for instance.
Also, maybe the enemies' casters could use more spells? I think they threw a pretty good amount of spells in PoE but I haven't really seen an enemy caster use slicken, for example. Many people here share the opinion that it's an awesome spell. Why would the enemy casters not use it then?
More terrain obstacles (clarification below)
I kind of touched on the topic above but having more obstacles (again, some places had this is White March. I don't know whether it was intentional or not) would be great and would make you use a lot more tactics, in my opinion. Trying to gain higher ground, so that you can further attack the enemy with your archers, for example, would be so epic. I don't really remember a game where this properly implemented.
Ice that melts or snow which slows you down and you can't quite go throw it very fast.
Whether / Terrain effects (clarification below)
One of the things which really bugged me about the end of White March 1 (no spoilers) was when you stood on top of something which should be very, very hot, but nobody of the group really reacted to it. Maybe that kind of behavior could at least fry your legs and that could slow your speed? The rain could maybe make you miss more with ranged weapons or maybe even melee weapons. I could totally see myself having a harder time to aim if my hands are slippery. This, for example, could be combined with the 2 suggestions above which would make for quite the epic battle, even if its a fight against weaker enemies. EX) Kobolds in watch towers with roofs or under rock which protect from the rain and who shoot at your party, while you're out and it's raining heavily.
The cold could also maybe reduce your reflexes (saving throws) or do other nasty stuff. Just some random ideas.
Armor / Weapon management (clarification below)
I remember my weapon in BG1 breaking quite a lot and it was quite annoying... but it was somewhat realistic, if I have to be fair. I can totally see armor and weapons break or wear off. Like armor could maybe become rusty if you wear it for too long and you always go cross rivers, go down the sewers or it rains on them. Or maybe you could reforge a weapon into another stronger version? Like they did with Andúril (the sword) from Lord of the Rings.
Poison / sicknesses / etc (clarification below)
One of the things which I really didn't like about IE games in general was poison... but it was kind of cool at the same time. I don't really think it would work with the current vitality / endurance system, but maybe it could at least
(Option to) turn off the combat log and turn on a descriptive log (clarification below)
The combat log is really very convenient. I completely agree... I sometimes check it when I'm unsure of something, and it was of great help but... I don't know about the others... but it would be so darn cool if I couldn't tell how much damage I was dealing and whether it was immune to something, unless I fight the same monster numerous time. That would make the bestiary, checking books on monsters ingame or asking NPC about a particular monster ingame a lot more meaningful, in my opinion. For example, a simple message like "your weapons don't seem to have an effect" instead of "the monster is immune to normal weapons" could either mean that my current normal weapons or not doing any damage or that my slashing weapon isn't doing any damage. If it's the first time I'm facing that particular monster, I would have to switch between weapons and actually see what really is working and what is not. Etc...
Looking forward to PoE2! I'll certainly play it, however you make it