I don't think the "feel" of the two shows would meld very well ...
however it's possible someone might dig up some more common ground to
make it work.

I think it would be a trifle more plausible if someone OTHER than Ranma
was the next slayer. Perhaps Akane (her strength is not unlike
Buffy's... and she use her (not-really-her-exclusive-thing-but...)
hammers to pound in stakes. Ukyo would be interesting as a Slayer.
Especially if she showed up and was taken for a boy. ^_^

Nightman