I think there's wayyy to many things to try to touch on so i'll just talk about the main things i dislike in D3 or even PoE that I don't want to see in D4
1. Set items and powerful legendaries. I don't necessarily feel that Rares should or could be better than legendaries should be the way to go. However, I think that they should not be mandatory like they are now. The increase to specific skill damages is just dumb. I think it's fine that legendaries rain from the sky. However, I think that they should just have interesting affects on them that could change how certain mechanics work. Instead of a huge stat boost that makes them a mandatory item. I think that every unique/legendary should be useful and not just junk. That's what the lower tier rarities are for. Seeing a legendary or unique drop should be really exciting.
2. I think that one thing PoE really got right was how they balance of power that affects your character's overall damage and survivability. I'd like to see more of this in D4 however I'd like to see an expansion on this idea. I think that Diablo attempted this with legendary gems and paragons. However, these are very 2 dimensional. There's very little choice when it comes to which legendary gems to use due to some being completely useless and do we even need to talk about choice with paragons?
3. Economy. I don't want to say PoE has the right idea but neither does D3. For veteran ARPG players poe's trading system is not complicated but can feel complicated to new players. Trading shouldn't be complicated. Trade should only take up a small portion of your time in the game. we should be out killing stuff and finding loot. Not sitting in trade chat or staring at an Auction house all day. I think an auction house could work however, the useable currency needs to be controlled better. It can't be so easily obtained like it is in diablo 3. It also needs multiple gold sinks in the game that prevents inflation. Also, markets can easily get saturated with people constantly finding gear. Having a system that also reduces the number of items on the market is good too. Something like Bind on Equip mechanic or even a crafting system that allows you to upgrade your item but every time you upgrade it has a chance to break.
4. Replayability - D3 has some terrible replayability. It if wasn't for seasons I probably wouldn't go back and play much. Even when the season comes out I don't play for very long. Same with PoE. I go back for every league but get burned out eventually. I like the diverse character building in PoE this is what causes me to come back and play multiple characters a league. I get the character to the level I'm happy with. do maps for a bit and if I really like the build I did then I push further. However, eventually I get burned out and leveling another character can seem annoying since you have to go through every act all over again. I think going through the story again to level should be optional, because if i could just level characters in PoE without going through the story again I'd probably play it a lot more. Because I do find making new characters fun for the sake of trying out different builds. In regards to D3, The whole being able to just switch gear and skils and now your'e doing a new build is dumb. This completely removed a whole level of replayability for many players (obviously me included). It's not even about being burned out. you just get bored because you've tried every build there is in the game and not everyone finds pushing GRIFT 130 fun. Every season they slightly buff a few builds. We come back and play a build we've already played before but maybe now we can push slightly higher in Greater rifts with it. In this PoE wins hands down, every time i go back to play the new league i make atleast 2-3 characters and have yet to make the same build twice. However, it's not perfect since I really only go back for the leagues.