How will new setting books be different in One D&D?
More experimental?
I think we're seeing the experimentation right now. The two competing options seem to be the adventure-setting hybrid book (Strixhaven, Dragonlance) and the three-book slipcase set (Spelljammer, Planescape). Whichever turns out to be more successful will probably dominate in the One D&D era. Both Strixhaven and Spelljammer have experienced some criticism, but the latter two books might do better. (If both models do equally well, we may see both after 2024.)
Will the Phyrexian arc merged the D&D and MtG multiverses making joint setting easier to make, as sort of foretold in 2021?
I remain flabbergasted that Wizards Live didn't end with the announcement of a Dominaria D&D setting next year. If there was ever a time to do it, it would be then.
Note there's space in the schedule at the end of 2023 for a sixth product, which could certainly be a D&D/MTG product that ties into the anniversary storylines. That could be a Dominaria 2.0.
Will Darksun, Greyhawk, and Mystara make a triumphet return in One D&D?
"Athaspace" becoming "Doomspace" either means Dark Sun has missed its best chance (because they decided it was massive reworking or nothing), or an indicator that a One D&D Dark Sun may appear sometime after 2024 (because they changed their mind about the revisions, and want to save it for later; note they also reference Athas in the playtest packet). I suspect the former, but can't rule out the latter.
Greyhawk is likely to return as a 50th anniversary adventure-setting book. (Dragonlance might serve as kind of a dry run for such a product.) But I'd be surprised if they make a full setting product for Greyhawk.
Weirdly, I wouldn't rule out some kind of return appearance by Mystara, because they keep referencing it even in the current transitional era. But I'm not sure how. Maybe a cameo in Planescape or something.
Will FR get a new edition Campaign book as is tradition.
I'm pretty certain that's the returning setting for 2024. Likely to be released alongside the new core. SCAG is extremely outdated and certainly won't serve as the baseline for the Realms in the One D&D era. (Expect some retcons while they're at it. But probably not a reboot.)
Will Theros, Eberron, Ravnica, Ravenloft,Spelljammer, Planescape, and Exandia also eventually get One D&D books?
Theros and Ravnica, probably not; neither were smash hits and they're technically usable with One D&D.
Eberron could be semi-updated with an adventure-setting book as
@Parmandur suggested.
Ravenloft, Spelljammer, and Planescape are all designed to be forward compatible with One D&D, so they won't bother.
Exandria is the big question. I'm sure Wizards would like to. But the CR folks made a point of releasing the updated Tal'dorei as an independent third-party product, and I imagine they'd rather do their own version of
Exandria Wildemount when the time comes to update it as well.