Parallel dimensions where the PC's are confronted with evil selves

[Stalker0]

There's a mirror in the dmg that does this. Just adapt its rules to a parallel dimension, and go from there. Its a great way to go, when your players have fought practically everything else.

 

[Khorod]

Might be interesting to play up on the Opposites Dimension.
The player doubles will be evil, but their archnemesis sends his lackeys to help them (lackeys which are all paladins or something in that world).

 

[Zhure]

Whatever you do, don't forget the goatees!

First let me respectfully say, "damn you for saying it first."

Secondly, that means I'm someone else's evil twin.