First of all I loved the game. Nothing scratched my infinity engine itch for nearly 15 years so this game was like a vacation with an old friend. I loved the story, the world, the classes, the look, even gathering my party before venturing forth. It came so close to hitting the highs of Planescape: Torment with its world-building and the epic feel of BG2. I think a few things could be improved upon for PoE2 that would really cement this franchise as one of the all time greats, if it weren't so already.
1) itemization - itemization feels strangely unsatisfying; many unique items are very similar, enchanting options don't allow to add some of the more unique effects, and in the late game all but a few items are highly fungible. In the early game this isn't so much an issue given the loot progression. Even the soul bound weapons don't quite scratch the itch; the process to level these weapons off sometimes requires the player to break immersion to act in ways he otherwise wouldn't (be revived in combat, etc...) and the class restrictions sometimes force weapons into the hands of less ideal characters. I think itemization needs an overhaul so that those few epic items really feel special, and soul bound items can be more useful to other classes. Perhaps in White March 2?
2) party member quests - party member quests often felt like they were missing key chunks. Without spoilers, all but a handful have a satisfying conclusion. While one or two cliff hangers or mysteries would have a great effect, most characters feel like they are left hanging. This is felt even more in the expansion where one of the new characters has a detailed opening exposition that jumps straight into the conclusion. I would love for more involved party quests. Sagani, for example, may have my favorite side quest in the game. It feels epic and that she's a part of it for most of the game.
Those are my main suggestions for improvement but I had a blast with my 100+ hours in Eora