Always there is somebody who wants to create new zones to can feel there is yet some unexplored region.
My idea is some zones of the infinite staircase are like the creepypasta mythology of the backrooms. I also want to recover the plane of the mirrors. This is not really the other side of the mirror, but this is the easiest way to explain. It could work like the "void space" where the main characters of "Dandadan" fight against aliens.
We could bet WotC wants to create a "Witchlight" setting about domains of delinght in the Feywild. My own suggestion is to add a new cluster of demiplanes within the elemental chaos, the Io's blood islands, domains ruled by the dragon overlords, for an updated "Council of Wyrns". These "islands" should allow space for all the type of dragons from all editions, for example a domain where the yuan-ti are ruled by cobra dragons.
And the "world tree" plane could work like the home of the wood elemental monsters.
Do you remember anything about the "spirit realm" in Kara-tur? I wouldn't like it was only a clone of the Feywild but with its own style, more focused into elemental forces, psionic and incarnum. It wouldn't be ruled by lord feys but "kami" courts. Do you remember the anime movie "Spirited Away"? Precious but not boring.