The reaves are too powerful to be a PC specie, but some spellcasters could a nerfect clone version. Maybe they are hired as mercenaries to stop an "isekai" invasion because their fading traits gives an effect like a time-limited immunity against ballistic damage by firearms.
The "vortex of Madness" could become a "mini-setting". It is too bizarre even for the standars of Limbo, but also the perfect place for intercompany crossovers. The Machine of Baron Lum the Mad could create a plot and a mindscape mixture of Matrix and "the Evil Within".
The demiplane of Invention/Leone could be "displaced" to Mechanus, a new regions be added later, creating a "cluster".
The "city of Glass" could be "cloned", in the sense there is one in the water plane, other in the fire, earth and air, like a mirage in different mirrors. It would work like an elemental version of Sigil. Maybe the PCs discover new portals to variants of the city of Glass, with magitek, and skyscrapper buildings, but also post-apocalypse by fault of zombie-like constructs.
* What about Dungeonland? and the shattered world.
Urunaland could be perfect if you want a D&D city-building videogame.
* What if the "mazes" are demiplanes created by the Lady of Pain, and later there "landed" in the infinite Staircase for the Sundering?
* Or the PCs are exploring the plane, and then the discover the Dungeonland+Land beyond the Magic Mirror. This could become a new setting style Ravenloft or Witchlight, where PCs explore "domains" based in famous franchises, for example a wicked version of classic fairy tales, or famous novels currently in public domain, for example stories by Julie Verne or Emilio Salgari.