Well if your going with an AU crossover RP, have some way for the characters to end up in location X from their Alternate Universe, and go from there.
I would quite like the idea of a Space ship, but that wouldn't work for the characters being the crew, given the fact that people want to be ORIGINAL with their AU characters, instead of making them the same character, just outfit for a space sip crew.

What it depends on is the type of crossover your going for (other than AU)
Action? [insert other option here]?