And the plane of the mirrors could be "retouch" to show variant and alternate paths. It could work as a "cosmic firewall" to stop possible invasion or conflicts between alternate timelines. And a true source of secrets could be a very interesting place for Vecna.
I would like to can add something like the spirit-realm from "Medium Videogame", the "otherside" from "Silent Hill" saga or the STEM from "Evil Within", Astral Plane in "Control" or the Labyrinth from "Remmant: from the ashes". The Shadowfell can be very creepy in the right hands but... Something where players felt to be within a creepypaste story. How would be a D&D liminal-space relaxing horror videogame?
When Planescape was published the outer planes were relatively unknown. Internet hadn't arrived yet. Today they are like "theme parks for planewalkers". Infernal planes are dangerous, but not so dreadful for players used to be the doom marine killing fiends with a shotgun or some Blizzard's Diablo. (But this isn't WotC's fault).
I am too used to imagine the infinite staircase like the "illusinory maze" from Jim Henson's Labyrinth, more "close-cell" like dungeons.
* Do we know how the other wildspaces were affected by the Spellplague?