While playing some other AARPGs, I had some thoughts on this subject. Maybe others do too. If this reaches the devs great, if not oh well.
Going from D3 to D4, there are only two things I want D4 to be:
More Solo Friendly
I just logged into D3 today (end of season) to check out my character on the LB, just to confirm what I am about to type out here: lots heroes with 120-130 clears, 3k paragon, 150 gems and 150 augments.
This post will probably be too long anyway so I am not going to go into great detail why this is atrociously prohibitive to any reasonable competitive solo player, but let me just point out that most people playing D3 probably don't realize just how big of a boost in power 10k+ mainstat is, not to mention the actual gem levels. For me personally there is a brick wall once I am in full augments and have pushed my build to its limits: start farming paragon/gems in groups or quit the season. You can guess what my choice usually is.
Bounties are a culprit here as well, but a very minor one. Again I don't think I have to explain this in great detail, but I'll add that bounties are just not very fun to do. If you keep a similar forgotten soul sink/reroll progression system, I suggest you make it a bit less tedious. I'm not being sarcastic btw; just tune whatever secondary progression system is planned for D4 to be a bit faster than what bounties are in D3.
Better Controls
Being forced to bind both left click and right click to skills, and being restricted in which specific skills can bind to what, is honestly the single most stupid design decision I have ever tolerated in any video game. It is inexcusable for this reason alone: you can circumvent this BS with macros, K+M alternatives and even controllers. Why you imposed such clunkiness on K+M players without the means or knowledge to solve this problem themselves is beyond me.
If you are going to be inspired by one feature of Chromaticon, please let it be the controls. Being able to bind movement to any key(s) you want is great, and being able to bind multiple skills to the same button is even better.
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With that out of the way, I also have some thoughts about endgame. Playing other AARPGs made me appreciate post-RoS D3's seasonal endgame a lot. I think that its pacing is particularly perfect. I'll give an outline of what I do every season and what I think works extremely well in D3. I hope this remains intact for D4:
-Leveling: 0 to 5 hours
-Getting Starter Build Online: <1 to 10 hours
How long it takes me to get a 6p together and whatever other crucial legendary is necessary to start farming at a decent torment level.
-Level Gems/First Augment: 5-10 hours
Here the real (and finally efficient) farming begins. First I level my gems and then I level my first augment. While farming and pushing to whatever augment level I decide on (this season it was 114 on my DH for 115 augments) I always get at least 1 well-rolled ancient I can augment, and so the ball starts rolling.
-Augmenting: ~20-30 hours
-Pushing
Usually it takes me 40-60 hours to be in full augments and ready to push. Of course luck plays a big role in how fast this is, how easy/hard rolling decent and augmentable ancients is for my specific build etc. Not to mention the LoD builds, which take much longer. Every single season though, this is a blast for me, and every season is a chance to play a class I haven't played in a while. This endgame is perfect imo, and I hope seasons play similarly in D4.
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Lastly I wanted to type out a few things about PoE, but this is getting too long. Just don't make D4 an economy based game. Feel free to draw inspiration from other aspects of PoE though, like the passive tree or labyrinth/ascendancy.