I really want some Planejammer action. I don't see both Planescape and Spelljammer getting separate releases, so why not portmanteau them into one thing? This isn't the first time I've mentioned this, but I do think it would be awesome. Just mash them up and pour a little Treasure Planet over the top and serve piping hot.
I'm not like, opposed to this, but how would it work? Because they have these fundamentally different approaches to travel, it's like Dr Who (Planescape) and Firefly (Spelljammer) being mashed up. They also have these fundamentally different settings. Planescape is all about the planes, Sigil, the Lady of Pain, the Factions, the Blood War, the Modron March and all these struggles over ideology and concepts. Spelljammer is much more trad/light-hearted, with villainous spider-eels and mind flayers, straight-backed, stuffy elves (standing in for the British Empire, I feel, somewhat), and all sorts of over-the-top stuff like the Giff and space hamsters. It doesn't really have a central conflict and it's all a bit unfocused.
I guess for me I'd basically want to overwrite a lot of the Spelljammer stuff with PS stuff. Like the Blood War could be fought with badass spaceships in hell-space (fits with the Mad Max deal in Avernus). The Modrons could have massive fleets of differently-sized and shaped and very precisely ordered ships, and so on. You could take the Neogi and Giff and that lot and make them into minor but memorable parts. Sigil becomes a massive space-station. People travel between crystal spheres with massive portals, over which battles take place and so on.
Hmmm I guess I see it could be done.
More precisely on-topic, what I want to see is:
1) Another Xanathars-style book.
2) More monsters. By all means many of them can be variants of existing monsters, but I love that lore stuff too.
3) Magic item and spells focused book. More spells, more items, more stuff on crafting items, including 2E-style awesome-crafting (where you have adventures to go and get stuff).
4) Planescape - I will accept a general Planar book if it puts the Factions back in Sigil and does Sigil some justice. See if you can find the cash to commission a couple of pieces of art from DiTerlizzi (maybe cover?). He's still amazing.
5) Dark Sun - Come on. And it should include a Psion and Psi or Dark Sun subclasses for all classes (Clerics don't need Psi subclasses, but they do need Elemental Priest and Templar subclasses).
6) Rules for level 20-30 play - could be in the Xanathars book, or in an epic adventure (probably fit the latter better and get me to finally buy an adventure).