Honestly, I never cared for Planescape. I don't hate it, like the Realms. I just never saw the lure. The slang was absurd. The Blood War seemed ham-handed. First level characters in the planes? Nuh-uh. And Sigil -- I just don't get it.
So, what I'd love to see... Find a way for me to use Planescape as an addition to my own campaign (home brew or, say, Greyhawk), instead of being in place of it. Who knows, I'm much more open to non-Tolkien fantasy than I was in the early 1990s. I might actually get into it, but I need a foothold to launch from.
Some of the suggestions others have made sound good, too. I've long had the devil/demon animosity be with the devils trying to take over and the demons realizing they're heading for oblivion no matter what so they just want to take as much with them as they can, so that one definitely sounds good.