Re: a planar sourcebook in general, I think the options presented show how difficult this would be to do right.
A lot of people here like the idea of a "toolbox" type, but that hasn't really been how 5E has been designed (despite optional stuff in the DMG, and the variable approach to cosmology), and whilst it would support more campaigns, I'm skeptical that it would actually be particularly successful or at all useful to most DMs.
I think what's a lot more likely is an approach which picks a cosmology and runs with it, and makes it a playable setting, not just a place high level adventurers go sometimes and low-level adventurers freak out about. What that cosmology would be is harder to say. Planescape is, ironically, more like a Magic setting than a D&D one in some ways. So you could have what some people have suggested and the ol' reverse tie-in, where they go with Planescape, and also put out a Magic set based on it around the same time or soon thereafter. I doubt this will happen because I strongly suspect the Magic team somewhat consider D&D's settings/IP material, and indeed that of pretty much everyone else, to be "beneath them". Also, arguably a Sigil-centric Planescape would kind of make Ravnica look bad, because Ravnica is kind of Sigil turned down to 6, and I really suspect the Magic team (who I think have been higher up the pecking order at WotC than the D&D team) would not appreciate that. I think they want to keep this strictly one way, which Magic stuff appearing in D&D, but not vice-versa.
That said, I might be completely wrong, the people running Magic may be very different to those of earlier eras, and with D&D's success over the last few years, things may have changed.
Personally I'm hoping for Planescape-centric, just think it's unlikely. Failing that, Planescape/Spelljammer mash-up. Failing that, a totally revised, modernised and re-thought Spelljammer.