Anyway, I was thinking more about this today, and I think that what I expect to happen with Spelljammer is solidly based on the nature of 5e, and 5e settings, and how new books are approached for it.
First, I think it will be more broad in scope. What I mean by broader in scope is that you'll be able to go to places in other planes than the material, like the elemental planes, the astral sea, feywild, shadowfell, etc. There will be spelljammer docks in the City of Brass, and quite possibly around Sigil. Fey-space will be interesting, and Shadow-space will be truly dire. I wish I could hope for something like the Deadlights from the Todash Darkness of Stephen King, here, but it'll probably be more "Gloomwraught, in space!" Which is also rad.
Second, I think it will also be broader in tone. The focus may still be goofy space opera high adventure, but I think the goofiness will be a bit more....optional, easier to ignore, like a lot of the silly stuff in 5e. This change will be much less stark.
Third, "space" won't restrict the action as much, so I don't expect the space gas to make things explode nor the spelljammers to block spellcasting. Any rules widgets from old Spelljammer that many players saw as arbitrarily restrictive and frustrating, will likely be reduced to either flavor text or optional rules.
Lastly, I think it is a certainty that parts of Planescape will be in Spelljammer, just as they have been in the core books, and are largely the basis of the whole edition. Planescape is the default setting of 5e, and has been since day one. The Sword Coast has always just been the Sharn to Planescapes world of Eberron. It gets the most attention, but the wider "world" is the actual setting. Now, I don't know if this means there won't be a book that is somehow just a Planescape book, down the line, but I really doubt there will ever be a Planescape book that isn't also a more general multiversal book. If we ever get a book that focuses on worldbuilding advice and tools for the homebrew worlds that are the majority of dnd play, it will be in the book that is also about Planescape. But the Spelljammer book will effectively be a lot of newbs' introduction to Planescape, or at least to many of the core elements of Planescape we haven't seen yet.
I think we will know whether there will be a Planescape book based on whether Sigil is mentioned in the Spelljammer book, and if so how much space it takes up in the book.