Really for me two major things I want to see above all else in the new Planescape:
1) The Factions.
I want them back in Sigil, doing stuff, being diverse, having actual beliefs, and being playable and intended to be played. They'd be much better as backgrounds than Feats, too (Piety could also work). What I don't want to see is that they're wishy-washy one-paragraph deals spread about the planes, and not really active in Sigil (which, unfortunately, is technically where 2E left them, thanks to Monte Cook, who claims he meant to fix it but WotC - who had just taken over - decided not to publish the next adventure in the series). And I can't stress enough them having actual beliefs - it's important that some of them be somewhat challenging or annoying to some people - because that's how beliefs and worldviews are. Not all the Factions need to look like heroes in anything but their own minds (and some not even then).
I want it to be weird, wacky, colourful, have its dumb slang back (just don't use it if you don't like it!), and to have a lot of detailed locations and organisations and business and just all sort of peculiarities and odd little details. Keep the quasi-Victorian tone - it's actually more in-tune with modern fantasy than it was back then - indeed I think it was influential on both RPGs and fantasy writers. Also I pray that they can avoid making it twee. It doesn't need to "dark" or "menacing" or whatever, but just enough edge to prevent it from being twee would be great. The LoP and her minions can help there, as can the presence of some of the not-very-nice Factions.