Highlander would work, but I'd really like to see a Buffy (or Angel)/Highlander crossover more than a Trek one...
I started a Highlander/Crow: Stairway to Heaven crossover some time back.
Lorraine Anderson wrote:
I've written fan-fic crossovers, and I'm in the middle of one right now. It's awfully hard to do both universes well -- and have them work equally successfuly in the solution of the problem. Also, my crossover characters tend to sit around and talk to each other a lot, until even I'm ready to tell them to get up and do something already!
I think of the "talking" stuff as a way to get used to the characters interacting with each other. Once you're comfortable with that, the action scenes will come later.