<<Ranma Saotome as the alien who ends up engaged to an earthling.>>

The engagement part fits, but I can't see Ranma in tiger stripes.  (Maybe
Ranma-chan ^_-)  Besides, Ranma isn't in the same league as Lum-sama.

Maybe we can keep Lum as Lum and Ranma as Ataru.  Ranma is maybe not a
pervert but we can be sure that the others girls will be pervert enough
for
him. 

Just an idea.

Salut bien !