I've recently watched an old Jon Pertwee-era Doctor Who episode which features - dun dun duuuuum! - a parallel universe with alternate, military dicatorship versions of everyone except the Doctor. You know, the whole Mirror, Mirror thing. Eyepatches, uniforms, the whole bit other than beards.
Back in the early nineties, I ran a 1st (and later 3rd) edition GURPS game set in the Forgotten Realms. We used the boxed set plus Waterdeep and the North, plus the City System boxed set and one or two other books. The campaign eventually moved from the inner sea area to Waterdeep and stayed there.
There was a recurring enemy . . . I think his name was Jarrick T'Jellan*. He was a nefarious type scheming for power, opposed to the good orders of the campaign world. The PCs fought him a few times, but he could Teleport and wouldn't stick around to die when defeated.
I'm not sure how I was going to set it up anymore, but I had planned to launch a Mirror, Mirror adventure. Toss the PCs into a parallel Waterdeep, where their only ally would be heroic rebel Jarrick T'Jellan. They'd get to fight alongside him, against the Lords of Waterdeep. We'd get to use those maps and stats we had for the good guys as maps and stats for the bad guys. They could knock off some of the Lords and fight a nice big battle against the evil overlords of the city. It would have been a blast, potentially.
Have any of you guys ever done this?
This is part of the fun of having the PCs fight alongside their foes in Felltower. From the moment they surrendered I knew that was going to be a thing. There isn't any point to a parallel megadungeon in my game, but there might be in yours. The fun of role reversal is a enjoyable. It might even be a chance to play Team Evil** for a change. Maybe bring the evil parallels of your PCs into the game world to do something nasty, and then have the PCs try to clean it up. Or play a switcheroo if the players are the type to like that*** and have them run their normal guys against evil, only to pull the curtain back and reveal they were playing an interlude of their evil guys. That would prevent any deliberate smoothing of the path of their real guys later.
I can see all sorts of fun. It's fun to watch, given well-established characters, and I'm sure it would have been fun the play, given the well-established villain of my previous campaign.
* I can't find my old paper notes, and my electronic ones are written with the old Wang word processor aka Leading Edge Word Processor and I'm not sure what will open them anymore. But I'm 90% certain on that name.
** See also.
*** I'm categorically NOT.