I guess a future setting wouldn't be only a world or planet but a wildspace or solar system. Maybe it would be a cluster of wildspaces, with the option of using "stargates" to explore other worlds.
I think you are right that a D&D world setting needs to think about the wildspace system that is in. Then the nearby moons and planets might also be an origin for certain species and cultures.
We should keep the idea of a possible Dark Sun spin-off where we can "borrow" our favorite elements but allowing creative freedom for player options and monsters.
2024
Dark Sun can present player options in three separate sections. During session zero, the DM determines which sections are in play.
Section 1: strictly old school Dark Sun except without slavery. Tyranny, environmental nihilism, and desperation of poverty and famine still apply. This section is a mini Players Handbook with the relevant species, classes, subclasses, and backgrounds, fully rewritten for the Dark Sun flavor and context.
The only species are:
. Human
. Elf (Athas lineage = Wood mechanically for speed but flavors for wasteland nomads)
. Dwarf
. Mul (= 2024 Orc mechanically for relentless but flavors as very tall statuesque Dwarf, and can only reproduce if by means of magic)
. Goliath (nicknamed "half giants")
. Halfling
. Thri-kreen
The only class options are:
. Fighter subclassing Champion, Psi Warrior, Battlemaster, and Gladiator (either as new background or new subclass)
. Rogue subclassing Thief, Soulknife, and Assassin (with "Troubadour" musician background)
. Ranger subclassing Hunter and Shadow
. Druid subclassing new "Preserver"
. Sorcerer subclassing new "Defiler"
. Warlock subclassing "Templar" Sorcerer-King patron
. Psion class (hopefully published first for generic D&D, and afterward Dark Sun also referring to it)
. Cleric subclassing new Elementalist domain (alternatively this is instead Druid subclasses of Land, Sea, plus new Fire and Air, and the Preserver relating to plant)
Surmising from the UA Apocalyptic Subclasses: the 5e Sorcerer which didnt exist in 2e now represents the 2e Defiler Magic-User, and relates conceptually to the defiling by the "Sorcerer" Kings. Preserver magic equates to the primal power source, thus the 5e Druid class appears to represent Preserver magic. As such, the strictly old school section doesnt offer the Wizard class as an option, since it is the 5e Sorcerer that represents the 2e Magic-User, not the 5e Wizard.
These section 1 player options define and curate the Dark Sun setting.
Section 2. This is an optional expansion of player options. It briefly recommends how to reflavor certain species and classes to make sense in the context of the Dark Sun setting.
Additional species mentioned include: Dray ancestry Dragonborn, Aarakocra birdfolk, Pterran Lizardfolk. I consider 2024 Goliath as a defacto Goliath-Genasi merger, but Genasi as a separate species from the Elemental Planes can make sense too.
The dwindling Dark Sun regions of "Positive Material" are Fey Crossings, whence Fey species are possible if rare, including the Eladrin lineage Elf and maybe High lineage on the Fey side of a Crossing.
The expanding "Negative Material" regions are Shadow Crossings, whence possible playable Undead species and significant Shadow and Necromant options. The "Gray" is Shadowfell. The "Black" is the Deep Shadow where the domains of dread exist including the one that imprisons Rajaat the first Sorcerer. Shadowfell is surprisingly important to Dark Sun flavor.
Epic boons transform into a Shadow Sorcerer-King or a Fey Avangion.
Additional classes include: Barbarian Berserker and Beast that might relate to Halfling culture; Bard understood as a psionic class with Valor and Glamour; Monk Hand, Elemental and Shadow; Ranger Beast and Fey; and possibly Paladin Fey Ancients and Vengeance.
It is tricky to introduce more Sorcerer and Warlock subclasses because of how Defiling works, and same goes for the Wizard class, but Shadow necromancy and Fey illusion can make sense thematically.
Section 3: This section offers environmental hope. Unknown to the towns of the Table Lands, there is a subterranean civilization around aquifers, elsewhere on planet Athas, working successfully to replenish elemental Water. They defend against those suffering waterless desperation but try to be responsible where they can. The civilization is advanced magitech, with Positive Material Fey allies.
Sections 2 and 3 are a smorgasbord of player options. The DM determines some or all of them, if any. The DM can also add whatever options one feels like.
A setting can't be only like a "snow globe" or a electric toy train always with the same railroad where the children only can watch. It has to be interactive and the players to feel inspiration to create their own stories. A setting has to be designed to allow multiple spin-off.
Pretty much any kitchen sink setting can spin off in any direction. The DM needs to remember that during session zero, it is just as important to remove player options as it is to add new player options, in order to define a new setting.