A couple ideas...
-Ranma/Sailor Moon. Before you scream at me for proposing another fukufic... what if Ranma was chosen instead of Hotaru as the 'host' for Mistress Nine? Even better, what if something went horribly wrong, and whenever splashed he now actually TURNS into Mistress Nine... separate personality and all?
To make it even more fun, throw in the stipulation that neither half has knowledge of what the other half was doing. Possibly, make the locking ladle useless due to interference from Mistress Nine's negative energy, so no cheap 'Nine gets the ladle and locks Ranma out forever'.
Next...
-Ranma/Excel Saga. Through a series of unfortunate accidents (namely, attempts to either kill or recruit him into Across and/or the Municipial Force Daitenzin), Ranma finds himself engaged to Excel. And Hyatt. And Elgarla. And Ropponmatsu. And if you feel truly sadistic, to Iz-Chan.
I don't think I need to tell you how much utter chaos such a scenario can cause. Sinc Excel Saga is a totally nonsensical series, you don't even have to make something serious up. Just go crazy.