The Mirror Plane mentioned in the 3e Manual of the Planes and under the Nerra entry in the Fiend Folio, existed back in 2e Planescape with the adventure dealing with the Kamarel in Tales of the Infinite Staircase.
But I currently don't like how the Plane of Mirrors are as it's described in the MotP (and the somewhat contradictorily mention in FF that time is static there when the MotP says it has normal time). I get it's supposed to be a dangerous hidden plane, but it gets out of hand when there's too many mirror duplicates produced. Especially every time you go to a different Mirror constellation. In my opinion the mirror duplicate should only be dealt with once and never appearing again once defeated, unless the Nerra actually capture someone and make a new duplicate.
Secondly the Plane of Mirrors needs to be more than just corridors of windows to other planes. Somewhere the corridor constellations have to lead to some sort of "Deep Mirror Plane" a universe of panes of some sort of reflective material suspended in a sparkling void. Entered from a doorway hidden in each constellation, where the exit resembles a shimmering pool of quicksilver. In the Deep Mirror Plane bitter nomadic clans of the Kamarel travel from pane to pane and the citadels of glass, crystal and almost solidly viscous liquid from the kingdoms of the Nerra stand...
Here it should have the traits:
-subjective directional gravity
-flowing time (haven't decided on the rate)
-infinite size
-magically morphic (shaped by Kamarel mirror magic mostly)
Destroying your mirror duplicate will draw the attention of the Nerra. However while mirror duplicates come into existence the first time you enter plane of mirrors, they can only exist in the constellations and not the deep mirror plane. The exact nature of the relation between the Kamarel and the Nerra aren't completely known, but they do seem to coexist without great incidences of violence.
Due to the nature of the plane, it's generally a secret plane to travel through, as it's been spoken of as being a very cursed and dangerous place, its mention only spoken of in hushed tones. Corrupting those who have went there and came back (probably because they were killed by their mirror duplicate).
I like to think of the mirror duplicates as alternate material plane versions of the PC. They entered the Plane of Mirrors the same way and at the same time as the PC because in their alternate reality they were doing something similar. It just so happens that only 2 alternate selves (the "original" PC and the "mirror duplicate) will ever enter the same section of mirrors at the same time.
The whole idea of spawning a new sentient being, fully equipped, with the sole purpose of replacing the PC just doesn't make any sense to me. Where did the material to form this mirror duplicate come from? Why does it have some kind of premonition of the whereabouts and a drive to kill the PC?
With my idea it is just as likely the PC and alternate reality PC would run into each other as it would be that they had their own adventures behind the looking glass and returned to their material plane without every running into each other. It also opens up the possibility of meeting multiple alternate selves while limiting the total number of alternate PCs to the number of nearby mirrors in that section of the plane of mirrors.
Just some ideas I thought I'd throw out there,
420