I'm simply spouting this off the top of my head, so bear with me...
I had a quick idea to get my players (such as they are at the moment, but that's a different story) into the Mirror Universe somehow - transporter malfunction, new technology, subspace anomaly, you know the drill.
I haven't decided whether to let them keep their ship, but regardless, the twist is that Q engineered their journey. He has been keeping an eye on the Mirror Universe, and has seen what will happen if the PC's don't intervene in a certain situation.
He shows them this - worlds destroyed, populations wiped out, general nastiness to the Nth degree, in both universes.
BUT...
The catch is that they have to work with their alternate selves in order to stop this catastrophic event from happening.
They will be forced to work with the sides of themselves they hate, everything the despise made flesh, in them. I really want to see the sparks fly.
What I need to know is:
1) Any ideas for a Universe-spanning catastrophe that can be averted?
2) Is it feasible for the players to alternate between their two selves?
3)Why would Q pick this particular bunch of people?
4) Any other comments or suggestions.