Oh I'd grab the book anyway (for ideas), But I based my one Mirror adventure on Diane Duane's 'Dark Mirror' novel.
In short: The Terran Empire had not fallen. All races had been crushed under it's boots. The Klingons were slaves. The Romulans were exterminated. The Cardies were exterminated. The Ferengi were treated as animals. And the empire kept grinding on like the juggernaught that it was.
One small problem. They'd taken over the Galactic arm they were in and it would take 20 years (at least) to cross into the one next door with present warp drive. You can see where this is going....
So they set their sites on our little dimension and are trying to develop crossing over technology. Nasty nasty nasty idea. And they are all highly competent and completely evil.
This could be fun....
A brave little theory, and actually quite coherent for a system of five or seven dimensions -- if only we lived in one.
Academician Prokhor Zakharov, "Now We Are Alone"