Let's remember always there are players who want to add their own "exotic ideas", for example an updated version of psionic ardents/divine minds as templars, or the totemist shaman (who adores some kaiju), or a sorcerer who primal magic instead arcane, or a "crusader" with "ki" martial maneuvers (Tome of Battle: Book of the Nine Swords) fighting like an arena gladiator, or even a D&D version of the biohacker class from Starfinder (do you remember the flux adept, osteomancer and the fleshwarper as 3.5 prestige classes?).
The land of the dead can be an interesting place "to be visited but not to live there". It may be a cool place for a couple of adventures, but a saturation of undeads in a campaign could cause a final reject.
From 4th Ed I love the idea of the "Land within the Wind" and the Athasian genasies.
The "Gray" could be an interesting place for a "crossover" with Ravenloft about Kalidnay. Maybe the souls of the sentient beings become "petitioners", outsiders keeping their memories and skills.
I like the idea of the Athasian giths as a new subrace, and the hidden faction from Black Spine module like the reason to explain because the travel through the Astral Plane is not possible or easy.
Some players may have got weird ideas about living technology created by the life-shifter raul-thaun, for example crossbows with artificial muscles to reload themself. (Do you remember the gristle gun from eXistenZ movie?)
Let's imagine what would happen if sorcerer-kings could find Yuuzang-Vorg biotechnology for reverse engineering.
Other idea I would like to add to my game would like something like the dread dust of Metal Gear Survive, like a zombie apocalypse but elemental-touched instead the classic undead, with the "gray glood" with failed elemental magic.
Maybe there are soma planar gates in Athas, but most of them toward the wrong adress, even one of them linked to the Far Realm, if you want Lovecraftian kaijus in your game. Do you know Lovecraft story "the mound"?