I think I explained my problem with the original Planescape well here:
I'm not sure what you would want from a 5e planescape. You've got almost everything you need mechanically and the 2e setting material is easily available. Except for the Planescape Monstrous Compendium Appendix I, II and III, none of which are available on DMs Guild, for reasons that I can only...
www.enworld.org
In short:
1. The factions are based on philosophy or ideology, but they don't have goals.
2. There are both too many similar factions, and the only factions people are familiar with are the original set so the later stuff will probably just alienate people.
3. Portals kind of suck because the players just feel railroaded. I called it "the World Serpent Inn expanded to a city" and if you've run or played any module featuring that you'll understand why that's railroadey.
4. The Lady of Pain is not a good character for a campaign. She has no goals (that you know of) and can't meaningfully be interacted with. She's there as a contrived cosmic cop, but otherwise it'd be better if she weren't there.
Overall the original setting felt like a static design as fixed as the Great Wheel, not a functioning world. I know the Faction War tried to change that... but of all the Planescape fans I know, none of them care about that.
That means I want:
1. Factions need distinct goals, not just philosophical perspectives.
2. Only Factions from the original set. No new ones. If you need to destroy or merge a few, okay, but keep it as original as you can.
3. Make portals work more like ports instead of having endless portals in the city. Fixed, known, and able to be taken back. Honestly, I kinda like the idea of using spelljamming ships to cross the Astral Sea to the outer planes, and I say that as someone who doesn't really like Spelljammer.
4. Do something else with the Lady of Pain. She's iconic, boring, and difficult to use. That's criminal.
5. We need descriptions of all the planes including adventuring hazzards and features, and the gate towns on each plane need descriptions. Yes that's obnoxious, but it's what you signed up for.
6. Most of the planar races have been covered, but I'd like to be able to play as an Abishai.
7. As a DM, I want a discussion about HOW to use the outer planes. I want ideas for using ideas, morals, ethics, and ideals as CHARACTERS in my adventure. The outer planes are all about the esoteric and surreal made manifest. Tell me about what that means and how I might take advantage of that.
Basically, I don't want Sigil to feel like a holodeck episode of Star Trek, but I want to be able to run adventures on planes that do feel like that.