I picked Planescape, Darksun, and Forgotten Realms.
I picked Planescape because they kept taking about the Multiverse, a clear hint, Planescape is a Tier 1 setting in popularity, and it's a chance to unify things and deal with some glaring omissions in 5e like no manual of the planes and the 3 or more missing Celestial races (mainly Archons, Guardianals, and Celestial Eladarin, but also possibly Asura and Illends).
I picked Darksun because someone else made a good point that lead me onto a chain of insights that made me realize we're likely getting a UA linked to it next month, before it gets released next year, perhaps in the Spring. Darksun is also a Tier 1 setting as I'd every other classic setting they've published so far.
I picked the Forgotten Realms because I think 2023's classic setting is the revisit Ray mentioned before, so all the reasons for am FR revisit are in effect. It's also classic setting, older then most of the other Tier 1 settings in fact. Also the timing is crazy good synergy with an FR $100,000,000 movie and BG3 full release coming out in 2023. Plus they will have so much Forgotten Realms MtG art by then that the art costs for the book will be nearly nothing.
Most of all those 3 picks properly will complete Tier 1 in popularity Setting Books for classic settings as Eberron and Ravenloft are already out.
I don't think it will be council of wyrms because Fizban's as close as we are getting to council of wyrms.
I don't think it's Spelljammer, because Spelljammer is the cameo.
Greyhawk will be saved for 2024.
Really I dare anyone to refute my logic.