"underdarkish" will be a disaster. Khyber is massively different from The Underdark but because of a stray "eberron's version of the underdark" or similar in one of the early books it's been an excuse to force FR lore and all of the incompatible baggage it carries into games where a gm is trying to make a clean break with a viscerally different setting. One of darksun's strengths is hat it does not have an underdark & even the gith colony stuck in the still semi-advanced blackspine(?) mountains module are very much not underdark.How do I hope D&D treats classic settings, going forward... Hmm. I think I'd like to go setting by setting on this one, Phil, with a single caveat:
All of them get updated to more modern sensibilities. More diversity of characters, less chainmail bikinis.
1) Dragonlance.
War of the Lance is the definitive era of Dragonlance. Oppressive police state, divine magic all but gone, magic users carefully ordered by society with heavy presumptions based on robe color. Good and Evil tangible and real -effects- upon the world, with straight up Good and Evil dragons in a hilariously blatant dichotomy of color coordination. I think that's perfect for Dragonlance.
Because at it's heart, Dragonlance is a child's view of D&D. I'm not saying you have to be a child to enjoy it, or anything. I'm saying that it's literally right there on the cover of a 5th grader's idea of fantasy: Riding dragons into battle like horses and wearing gaudy and awesome armor, fighting against a clearly demarcated evil which has a cosmic stamp of Red Lasers means Bad Guy type of morality.
Good is here. Bad is there. Go get 'em, champ!
LEAN INTO THAT.
Don't try to make it more complicated than it already is. Make it the simplest version of D&D narratives with the only moral complexity being interpersonal relationships against the backdrop of this horrible situation. Where a criminal type character might help the good guys even though they are, themselves, "Evil" because it sticks it to the fascist cops.
And then -market- it like that. Sell it to the youngest players. Aim for 8-10 to be introduced to Dragonlance through kids storybooks and comic books. Make a story about a young girl who finds a wounded little dragon and who grows up with the dragon as her bestest best friend and who eventually gets to be a knight and ride her horse-sized dragon into battle to save the world!
2) Dark Sun
It's taken me a while to come around but... Revamp it entirely.
Dark Sun is Sword and Sorcery with Weird Science tied into it and an Ecological Horror plotline. Adapt it for the current climate crisis and political allegory and move forward from there. Sure, keep the Thri-Kreen and the Sorcerer-Kings, keep Psionics and Defiling. But turn Magic into the full on Oil and Coal that it is, narratively.
Rather than just having magic only destroy, have it objectively and narratively pollute and corrupt. Change the setting from "All Desert" to "Hellish Mix of Landscapes". Yeah, Magic can destroy the world by ripping away all the vital and life-giving elements of it through desertifaction, but how about it also turning beautiful lakes into mire-valleys of thick mud, obnoxious insects, and poisonous water that glistens under the red sun that cannot be consumed without killing you?
How about a whole "Underdarkish" area where a Githyanki Sorcerer-King keeps ancient horrors in check beneath the world and her subjects toil in blackness to try and find the remnants of the collapsed portal or crashed spelljammer that brought their ancestors to this world, so long ago?
How about creating a magic-fuel resource in the world that can be used to keep spellcasting from harming the world... but it's only found underground and the uncaring Sorcerer-Kings employ Strip Mining to try and get every ounce of it for themselves so they can cast spells without destroying their city state in the process?
Keep the Giant-Striding Silt Sea with Skiffs on Skis and giant wheels that reach the bottom, obviously. Keep the massive tracts of wasteland. Keep the red mountains under the Olive Tinged Sky... just do more -nasty- landscape variety.
And the Political Allegory? Why stop there?!
Instead of Tektuktitlay as an Aztec God-King stand-in whose hopes of conquest are kept in check by Hamanu the Lion we go for a contemporary allegory instead? A petty tyrant far too weak to be a challenge for any of the other Sorcerer-Kings who talks a big game and uses a strong army to protect himself while he starves his people who sometimes escape to a nearby city-state to tell tales of the misery and hardship they've left behind, but occasionally change their tune and return to their city-state for the sake of those they left behind... His Templars enforce a form of thought-policing on the populace where any sort of divisive speech results in your neighbors turning you over to the state for Punishment... or Re-education.
And then come up with a new Metaplot beyond Tyr. Set it up more like Conan than Frodo. You're not the hero who is here to save the world, that ship has sailed. You're the hero who saves the sacrificial victim from the altar, kicks the villain's Lieutenant into the firepit, and then flees the reprisal while the sacrificial victim swoons over your big muscles and lustrous hair while exploring his own backstory as the descendent of some important bloodline whose sacrifice would be particularly powerful. Then you fight the horrifically scarred Lieutenant you set on fire before killing the High Priest in a climactic battle. Finally you take your new companion and/or lover to a seedy tavern and wait for the next High Adventure.
Oh. And use regional maps that don't connect, obviously. Gives DMs space to add stuff and makes the world feel disjointed. Love it in Ravenloft.
Aggressively market it to late teens and young adults (16-25) with short-story compilations and novels written in a bold and prozaic style that is very evocative of details.
3) Spelljammer
Planesjammer. Seriously, just do it. Make Sigil into a cool place for getting between different Planes of Existence, make Spelljammers the way you travel across starscapes to planets and do your whole schtick. There's no reason the two can't share the same universe and honestly mixing the flavors can be really nice. Make Sigil how people get from place to place for diplomacy, espionage, adventure, have Spelljammers be how goods and stuff move from place to place in bulk, and then let Githyanki Pirates attack those trade vessels and explorers and stuff.
Make the Spelljammer stuff a bit more serious than it was, to play into a more teenager vibe, but with plenty of room for the quirky Saturday Morning Cartoon play. Then market it to 13-17 year olds, specifically. Young Adult novels and comics, crossovers with Magic: the Gathering, the whole shebang.
4) Birthright
Just ... uh... y'know... Make things less patriarchal and have at, I guess? There's really not a whole lot about this setting that warrants serious consideration for change. It's very simple and straightforward and it basically just works.
5) Greyhawk
Lean hard into the He-Man vibe with Iuz. Like Dragonlance, mostly market it towards kids and young teens. Have it kind of take over from where Dragonlance leaves off to introduce more D&D-centric concepts like Pantheons of Gods rather than dualism, Orcs and Owlbears, the classics that Dragonlance doesn't really "Do" as core conceits.
Sticking with the other themes you mention it would be better to just toss a more advanced colonial power out deep in the desert with the motive of being able to do things they can't do back home while basically running forced labor camps with plentiful food/water shipped in from another sphere & former remote villages who never want to leave instead of that uderdarkish thing. Put it deep enough & it doesn't even need that much of a presence. Sure the workers can leave, but a mindrape with new memories that purge any recollection of the location or wonderful bounty in a place with the worse abuses of megacorps & such solves all kinds of problems. They just want to be left alone & might pay off some of the SKs with occasional water/food shipments to keep them looking the other way but other than doing their thing they are just happy to keep using a banana republic that doesn't care what they do.