Forgotten Realms has become almost the default game world for D&D 5E. Ravenloft has also already been brought back. I am not sure to what extent I would say Greyhawk ever went anywhere, but frankly-- it felt like Greyhawk never had anything unique or special to it anyway that would make it worth bringing back.
Bringing back Al Qadim, Kara Tur and Maztica is just asking to be canceled for serious racial insensitivity-- you think things are bad with the orcs and drow? You want to bring in one of those settings that just unabashedly describes non-white cultures as "mystical" and "barbaric" and "foreign" and "mysterious"-- you know, out and out discounting people who came from the cultures that the elements are based on and are familiar with them remotely being in the audience... and then there is the whole thing that when dealing with any of the European-based settings they don't ask you to consider feudal era values, ethics, norms and cultural expectations, but the moment it is a setting where the default humans are non-white? Well-- then they got to emphasize how those there don't think or feel like "normal humans" and ask that you entirely actively work to shift your perspective to being what an outsider's opinion of what the people on that region thought and felt 500 years ago.
You really want to dangle that out there as something WotC should dive head-first into the deep-end of?
Dragonlance is fundamentally baked out of 1970s-1980s Saturday morning cartoon fantasy tropes and was so inherently tied to an overall metaplot that ended that there is really no reason to bring that thing back except for the nostalgia of 40+ years olds. It would be SO much better to examine the fantasy cartoons of past 20 years and swiping ideas from those to make into a new cartoony setting. There have certainly been no shortage of materials to work with.
Dark Sun was primarily inspired by Dune and maybe the Mad Max films being popular during that time. If that was going to be turned out, it should have already been done. Besides-- I am not sure how the whole "survival horror" aspect would translate well to the modern era. And that's not even considering the issue of the incredibly overpowered setting specific races that I am not even sure how you could even begin to balance. Still-- it is way better than most of those options.
Birthright was a setting primarily only designed for a kingdom-building game and it was never particularly popular in the first place.
Both Blackmoor and Mystara are generally so super bland that I have no idea what you would even want to go back to-- possibly even more so than GreyHawk. Sure, I guess there were a couple supplements that offered a couple interesting aspects to the settings, but nothing that couldn't just be swiped and brought into a new setting with generally more interesting elements.
Really-- Planescape and Starjammer are the best choices of these, and probably only because you can do literally anything imaginable with Planescape and Starjammer is semi-sci-fi which inherently brings in a new element.
Forgotten Realms has become almost the default game world for D&D 5E, as GreenTengu already wrote above. However, there is no updated map of the entire continent or the other continents. If you want to play anywhere outside of the Swordcoast, you are stuck with maps of 3E/4E, which is made really confusing because the Spellplague changed those maps a lot. Newer players (such as myself) who never played 3E/4E have no clue about the Spellplague and cannot be bothered to piece together how the world may have become in 5E. I would simply like an updated map, and optionally some info on a bunch of interesting towns and read which countries are friends with each other and which are at war or at least hostile to each other, although I might rewrite that myself anyway for a new campaign.
And if, as GreenTengu wrote, the other continents are just a bunch of insensitive discimination, then leave that out, or rewrite it. It is in fact quite easy to describe cultures without discrimination. You can either choose to describe it such that it does not resemble a culture here on Earth (perhaps rename the continent too?) or just remove all discrimination and opinions. Easiest way to do that is to have a few campaign settings in which the heroes and good guys are from those continents and the barbarians and savages are from elsewhere. I'm pretty sure there are plenty savages living on Faerun.
So many people are taking the news and assuming that these will be fully baked setting accessories with as much material as the old boxed sets packed.
It could be one particular aspect of one type of play a setting was known for and base an anthology of adventures around that.
For instance, Mystara has immortals that were former mortals, PCs have paths to immortality, and it has the Hollow World of past civilizations to visit. And as it used hex maps it was optimal for an exploration style game.
Greyhawk has the world war of that Gygax wrote about in his Dragon Magazine column that became Greyhawk Wars, followed by the From the Ashes period where evil had just won. A 5e product on building up to and running a war and the aftermath of evil victorious fits.
And Birthright was less kingdom building, and more being PCs who were leaders of kingdoms or organizations who then adventured. I can say I want this one the least because I don’t want completion to MCDM’s coming Kingdoms and Warfare book.
If you take out the kingdom building, Birthright with its various interesting human groups, tweaked fey elfs and halflings and the awnshegh as unique amd powerful monsters is a cool setting. Its also a much more grounded, realistic and even low magic take than most D&D settings, fillied with medieval politics, intrigue, and lore.
Tweaking the Kingdom building and Bloodright rules could allow for all varieties of play from standard adventuring party,Game of Thrones style politics, mercantile intrigues, immortal powers through to fully blooded rulers in a war game
I voted for Dragonlance, Greyhawk, and Extended FR. I think this means I want different takes on "traditional fantasy" that are still basically traditional fantasy. I'd like to see what tweaks would be made to emulate those flavors.
I have grown to the point that FR is basically where we play since 5e started. Nobody has the time to make a homebrew anymore and the time to plan is spent on adventures, not world building. While I do make towns and NPCs and such, the FR frame works fine for me. I would like to get some more of the world frame to develop though. I have a lot of older stuff and the web has almost anything, but some updates would be cool to help.
Even a greater Sword Coast would be good with some High Forest and the desert along with some more southern stuff. Not asking for places we never player in before, but Cormyr or Bloodstone would help.
I also voted for Greyhawk thinking it would be cool to see the 'more gritty' setting and how rules may change slightly.
Curse of the Strand will be remembered like a classic from the 5th Ed, adding new thigs there weren't before.
I have said Hasbro's plans for D&D are linked to multimedia franchises.
After Baldur's Gate III the next title could be a new Planescape videogame.
Spelljammers could appear as "guest artist" in Planescape sourcebooks.
Dragonlance will return, but not now yet.
After reading an article about a "eastern egg" in the last module, I dare to say we could be a multiverse crisis event in D&D, linked with Vecna (if I say this as theory it's not a true spoiler), and this could cause some retcons and even a almost total reboot of all the D&D worlds.
My bet is Dark Sun and Planescape, maybe Spelljammer and a new module as sequel of Curse of Strand. (here we couldn't talk about the return of Ravenloft because this has been published).
Dark Sun, for me, is definitely a slam dunk option. More Psionics, hopefully a full class, and several new races, plus a post-apocalyptic vibe, I feel like it's a different enough world from the other settings that have come out. This one I definitely want.
... As for Planescape and Spelljammer ... I want one, or maybe a fusion of them as a book on interplanar adventures. Given the increased integration with MTG (with more worlds getting setting books?), official support to run campaigns that can run on multiple planes seems like a great idea. I'll admit, I don't know much about the two settings.