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.