Since this thread caught my attention, i'll add my two bits to it. 

Ranma uses the Nabban Mirror to find a cure but instead found himself
in the Menji era. Meets Himura Kenshin... yata yata yata.. the end.

Much better ne?

Or... You can have Happosai transport Ranma-chan and himself to the
Menji era he can have his favorite student with him. With out the
usual complication Happosai force himself to Ranma-chan.

Oh and set it a the last volume with Happosai young.(eg he drank the
spring of the drowned man)

Guess who's the Hero of the Day?     

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