For campaign setting purposes, Planescape probably equals Sigil & the Outlands: everything else is just part of the standard D&D meta-setting, and is detailed in the DMG. If/when we see a Planescape product, I would expect it to be all about Sigil and the factions there.
I don't think if it happens will be Darksun yet, I don't think it's ready. I think it will either Planescape or Ravenloft.
This is true. And that's kind of the problem with Planescape: either you make it so big one big can barely scrape the surface, or you need to limit it to Sigil and the Outlands. If they do the later, I'd like to see it integrated into an Adventure Book rather than trying to justify a new campaign book for it.
For some context on the tiers of setting popularity, this is what WotC let us know about their survey responses in 2015:
"The popularity of settings in the survey fell into three distinct clusters. Not surprisingly, our most popular settings from prior editions landed at the top of the rankings, with Eberron, Ravenloft, Dark Sun, Planescape, and the Forgotten Realms all proving equally popular. Greyhawk, Dragonlance, and Spelljammer all shared a similar level of second-tier popularity, followed by a fairly steep drop-off to the rest of the settings. My sense is that Spelljammer has often lagged behind the broad popularity of other settings, falling into love-it-or-hate-it status depending on personal tastes. Greyhawk and Dragonlance hew fairly close to the assumptions we used in creating the fifth edition rulebooks, making them much easier to run with material from past editions. Of the top five settings, four require significant new material to function and the fifth is by far our most popular world."
https://dnd.wizards.com/articles/features/july-survey
I should bookmark this page and just refer back to it every now and then when I want to speculate or get a boost of optimism!
WotC has not moved overly swiftly, but they have acted in an upfront manner these past years, methinks.
Indeed. I wish they would release more, but I've come to terms with how they do things and just enjoying the game and speculation. I also really like reading the survey results, but I don't think we've gotten any in a while now.