Charles Rampant
Adventurer
I wish they'd just hit a reboot button and give us back DL, FR, Greyhawk, Spelljammer, DS with the original materials updated to 5E rules. An 'alternative universe' reset.
These settings worked best when they were introduced and before so many bandaids were applied to them. Take them back to the basics.
I wanted to quote this and ignore the comments about specific settings, since I think that there's a really interesting idea here. While I'm a fan of the depth to be found in the Forgotten Realms, there is a common observation around here that Greyhawk is preferred by some because it lacks that depth - you've got a lot more room to improvise your own details, there is more blank space on the map. It could be observed that many of the settings started out as fairly light touch, more conceptually interesting than super detailed in their own right, but that TSR then ploughed so much effort into developing them that they became overbearing. Dark Sun is not a setting I've read, but I know that it started with a well-regarded boxed set; it was then supported by, among other things, a book on caravans. This seems like the kind of thing that isn't really required; you could indeed say that leaving it at just the initial boxed set, and then having adventures that expanded on stuff as needed, would have been a superior model.
In addition, many of the settings are also obviously doing different things - Ravenloft is gothic horror, and suits a very different kind of adventure than Dark Sun, which is post-apoc fantasy. With the possible exception of the overlapping generic fantasy ones - Forgotten Realms, Greyhawk, Mystara - the settings are all useful to maintain, since Planescape lets you do different things as a writer than Spelljammer does.
Taking this together, we might see that Wizards of the Coast will look upon their currently defunct setting IP, and see a great way to do interesting and different products in the future, in a way that doesn't bloat the market and split their fanbase. Instead of doing a whole game line for Planescape, with a boxed set for every imaginable combination of planes, we will see an adventure that takes place there, or visits it, with the discerning reader directed to the DMsGuild and perhaps an accompanying winter hardback book (Shemeshka's Guide to the Planes, Mordenkainen's Guide to Magic) that contains just enough player-facing setting material to support it. Then if they fancy returning to Ravenloft, say, that's not a big deal; another adventure can be located near Barovia, with perhaps a few links, but without the need for a whole supporting line.