In one of Nathan Stewart's more recent Fireside Chats on twitch/youtube, he stated quite authoritatively to a chat question that Spelljammer was not coming this year (question asked because of all the planar foes in MToF). Which could mean a couple things-- 1) he was outright lying/misleading, or 2) it's not Spelljammer but rather Planescape that is the setting book that is the more likely planar setting book.
The way he and Mike et. al. have been talking all along, they've made it sound like game mechanical additions would not arrive officially until the was a product they could be connected to. So they wouldn't release the mystic until the released an official Dark Sun book, they wouldn't release the Artificer until they released an Eberron book etc. So we can take what we've been seeing in like Mike Maerls' Happy Fun Hour two different ways-- 1) even though he's been making UAs of ways to play spell-like psionics in the mean time until the official psionic products are released, Dark Sun is not one of the soon-to-be feature settings BECAUSE there is not official psionics at this point in time... or 2) the Mystic as a class with its full mechanical suite of psionics won't be released until an official Dark Sun setting tome is completed, but the Fighter/Monk/Wizard UA subclasses are being made specifically because they ARE making Dark Sun one of their first setting "openings" and want players to have something to use with them in the mean time.
Which way it goes is probably dependent on just how large and official-like the new openings are going to be. If they are being released as a full book with the intention that this is it for that particular setting, then my guess would be Dark Sun wouldn't be one of them. Mainly because they aren't far enough along with the Mystic to release it is this was to be the one and only DS book they were going to make. Whereas they could release Greyhawk right now with a one-and-done without needing any major mechanical work requiring playtesting.
Whereas if these "openings" are just digital info and DMsGuild releases for the time being, and there's still big setting books to come in years future... then opening Dark Sun (with the UA psionics to help out) seems more likely.