How about a Ranma/The World God Only Knows crossover?
Somehow Hell has found out that there are a lot of girls after Ranma and that he might be a great help at catching runaway spirits. Somehow Ranma signs a contract and he is forced to help Elsie capture the spirits.
However, there are severla complications.
1) Ranma doesn't know how to relate to somewhat normal girls (as it seems he's only attracting the violent and irrational ones). It will be a serious blow to his ego once he finds out that for the normal girls the world does actually not revolve around him and they are not interested.
2) Elsie being the naive girl she is will constantly cause trouble for Ranma as the fiancées will be in a continuous jealous mode because she's around Ranma all the time.
3) The fiancées must not learn about Ranma's mission, and unfortunately whenver they see Ranma flirt with other girls he's in big trouble.