@Ruin Explorer i do think you have a point.
Like a Full caster with mor flexibility and fewer constraints.
Both of those are relative to other classes. More flexibility I think you could reasonably argue. They had too many approaches they could use too early on compared to some classes. I don't think they were as flexible as a well-played Wizard, but they were certainly flexible. Less constraints? That's not even arguable. They had a bigger constraint than any full caster, that being their power pool, which given it was daily, wasn't huge, wasn't anywhere near as deep as a full caster. And that's where most claims of "overpoweredness" completely collapsed. Because they all relied on the power pool being infinite. People would say "Oh they can do X damage for Y rounds, every day!" and then also claim they were flexible and had utility, and it just mathematically, factually, unarguable was flatly false. They literally couldn't be both - they could do damage, or the they could have utility. Using one reduced the other. It's like claiming a Wizard is overpowered by having them spend every spell slot on the biggest-number damage spells AND also claiming they were overpowered because they could cast all these non-ritual utility spells.
Also as an aside, the funniest thing re: claims of "overpowered" is that everyone spent the power points incredibly inefficiently, just trying to show they were "OP" with "big numbers", rather than looking at how much damage and utility they could do over a 6-8 encounter adventuring day. If you actually did burn power like the "overpowered" analyses suggested, not only would you be out of power on like, the 1st, 2nd or 3rd encounter of the day (depending on how much HP the monsters had, and how much damage the rest of the party was doing), and have the same damage and utility as Wizard out of spells slots for the rest of the day, but your overall damage would actually be significantly lower than a Fighter or Rogue (even without subclasses, and yes fully accounting for misses etc.), let alone a full caster who got off some AOE spells. If you played it a very different way, and used your power ultra-efficiently, and had zero utility, you could catch up to a Fighter, but as soon as a full caster landed some AoEs, you'd be way behind, and if the fights lasted more than three rounds, or there were more than 6 encounters in a day, the Fighter and Rogue would pull well ahead again.
I think it was all pretty easy to fix even if you did see a problem. Drop the power pool by a fair bit, but of it back on a short rest. Limit the amount spent on any given power a bit more tightly. Give them fewer powers early on. Nerf/remove literally a handful of powers (including the single, specific power everyone used to "prove" it was OP). And that wouldn't even really be necessary, but it could have been proven to be balanced in a way no-one could have argued with (it would have actually be a fair bit weaker than full casters at that point).
Back on main topic:
I think that rather than a Planescape or Spelljammer book, as such, they will make a book that simply explores planar and world-hopping adventure, will have spelljammers, sigil, and other elements of those two properties, but the work will put the current 5e dnd multi-verse first, and any consideration of the canon of those settings a distant 5th place after fun, expansion of the ruleset, and whatever pet widgets they want to play with that work out in UA.
I don't think is quite as likely as you expect, but solely because spelljammers are potentially a huge subject, because of the implications that roll outwards from their existence. Sigil and Planescape stuff in general, by comparison, is extremely contained. The concept of portals is neat, precise, makes immediate sense to most people, and takes up like, three paragraphs, maybe a page if you're feeling luxurious and want to give loads of examples. Whereas just describing the basic functionality of a spelljammer is quite a bit of work, and then you just create a million questions, because do you have crystal spheres? What's between the stars? What about gravity or solar radiation or what-have-you? And then you need a lot of suggestions, and maybe even a default and so on.
I think astral ships (which, iirc, predate spelljammers) are a bit more likely, and potentially could fill a similar role to spelljammers without raising quite so many questions, and could merely be part of a (large) chapter on the Astral plane (which has been underused in recent editions, I feel).
