I don't know if anyone has done this, but how about a Ranma 1/2
-'You Got Mail' crossover?  Just couple up any two Ranma 1/2 characters
that hate each other as in the framework of the movie inserted into the
Ranma series. The only pairings that I see that could work this way are
Ranma/Nabiki, Tatewaki/Nabiki, Ryoga/Ukyo <--(a stretch, I admit. (since
there isn't really open hostility between them) take an artistic liberty), 
or possibly Mousse/Akane <--(Hmm...).