I voted as follows:
Planescape/Spelljammer hybrid - This is like, a no-brainer, and would be pretty easy to make great frankly, but I see one problem with it - it's too good of an idea for a corporation like WotC. What do I mean by that? Fans of various shows, movie series, games, etc. often come up with theories, sometimes quite elaborate theories, as to what they're going to do next. Long experience dictates though, that if an idea makes too much sense, is too cool, just adds up a little bit too well, it's almost certainly wrong, because the people involved have probably thought about it significantly longer, harder, and with more clarity than the people at the corporation will have. This isn't hindsight stuff I'm talking about either, it's anticipation. Planescape/Spelljammer hybrid is such a basically good idea that it sets of the "makes too much sense" alarms in my head. Generally the smaller the company making a thing, the more chance a really good idea like this is going to happen, whereas the larger the company, and the more money involved, the more likely they half-arsed the basic idea, because they had to get working on all the production elements.
So I think that it would obviously be a great idea, but we may well actually just see a Planescape or Spelljammer reboot that isn't a hybrid, and maybe isn't very exciting at all (esp. if they do something dim and go with "Planescape as Monte Cook - that criminal - left it" < loud sounds of axe meeting grindstone >). That said over a year ago DiTerlizzi seemed to be working on PS stuff so who knows.
Forgotten Realms - Faerun only - I wouldn't buy it, but an awful lot of people would, and I'd be surprised if we didn't see an FR book with a lot more coverage than The Sword Coast in the next three-four years.
Dark Sun - I think there's zero question that they want to do Dark Sun given the various references, mentions, and so on. It's one of the previous-edition settings with the most name recognition, especially from younger fans, weirdly (possibly because it was also a 4E setting but I think there's a bit more going on), it's a setting different to what they've done before, and the ecological elements and the fact that the players may be fighting for a very literal kind of justice against oppressive rulers make it feel perhaps even more relevant now than it was in the 1990s. I'd expect significant changes because fundamentally 5E's approach to magic is different to 2E, and WotC completely messed up their work on Psionics by applying irrational acceptance standards to them.
I know a lot of people are expecting Dragonlance because there's a Dragonlance novel or novel series coming up but historically neither TSR nor WotC have consistently matched DL novel releases with actual RPG products. Especially as one suspects the core audience for DL novels is now, well, mid-40s and up in age, so one of the smallest segments of the D&D audience. If you look on places where fantasy novels are discussed, I think you'll find very few people under 40 have read the DL novels, and even smaller proportion of those hold a positive opinion rather than "Yeah I read them when I was a kid then I tried re-reading them more recently and I didn't enjoy them much". Plus they're super-generic fantasy with a ton of problematic elements and WotC tried to not publish the most recent one! So I wouldn't be shocked if DL was one of the upcoming classic settings, but I would be at least a little bit surprised.
As for Greyhawk, I'm really not seeing it. I've been playing D&D since 1989. During that time there's been what, one crummy GH gazeteer for 3E, despite 3E being explicitly GH-as-default?
Looking it up I see there technically some adventures for GH and "City of GH" module released in 1989, then a couple of failed attempts to make GH happen in the early '90s (involving adventures again), before it got totally cancelled in 1994. Then WotC tried to make GH happen again in late 2E in 1998, with small player's guide and "The adventure begins", a small setting-guide which hilariously I've never even heard of seen before (so it clearly wasn't making it to the various FLGSes I visited at the time).
Then 3E had Living GH, via RPGA, but that seems to have reached a fairly tiny (if extremely dedicated) audience estimated by RPGA at 10k people (players and DMs), and despite all the work and literally hundreds of adventures, it's all considered non-canon by WotC. Since then we've had nothing but a couple of adventures where it's the default setting.
So I'm mystified as to who they'd be aiming at with a GH revival. I mean, I could see it happening - we've three basically-irrational attempts to resurrect GH, in the early 1990s, in 1998, and in 2000. None of those made any sense, none of them were successful, but it didn't stop people from trying repeatedly. Could that happen again? Even in the much-more-rational WotC of 2021 and onwards? I think it could. Ray Winninger worked at TSR in the 1980s, he's certainly old enough that he could have some serious nostalgia for GH (though judging from his own RPG output, I'd be slightly surprised if he did). It's particularly odd as an idea because the sort of people I could see it appealing to are extremely well-catered to by pretty high-quality and prolific RPGs like DCC and other OSR stuff. I mean, the FR has been steadily popular for 30-odd years at this point, and they've felt no reason to go beyond SCAG and various adventures, so it's particularly hard to see why they'd go "Hey this setting was popular like 35 years ago, let's make it happen again!". I could see a "maximum nostalgia" product for D&D's 50th I guess. They certainly did nostalgia stuff with GH adventures for the 25th (which was also WotC). But looking at the irrational attempts to make it happen, you never know...