Okay...  Here's my latest idea.  Anyone care to take a stab at it?

Ranma is sick and tired of Shampoo chasing him, trying to drug him, etc.,
so he decides to take matters into his own hands.  He offhandedly remarks
to Shampoo that Mousse has gotten a lot better lately and could easily beat
her anytime he chose, but simply refused to do so because he didn't want to
hurt her.  Shampoo, her pride wounded, says that Mousse could never beat
her no matter how hard he tried.  Ranma bets her that Mousse can beat her
and that if he does that she will leave Ranma alone.  Shampoo asks what she
gets if Mousse loses and Ranma agrees to go on a date with her.  Ranma
challenges Mousse to a fight, then during the fight, he (where Mousse can
see) dons a long purple wig and starts taunting Mousse in Shampoo-ese
("Stupid Mousse!  You no can win!  Give up!")  Then, Ranma runs off,
allowing Mousse to keep him in sight the entire time.  Then, Ranma bolts
into the Nakohanten with Mousse close behind and hides.  Mousse enters and
screams for Ranma to show himself.  Shampoo emerges from the kitchen,
demanding to know what Mousse is doing.  Mousse, mistakes her for Ranma in
disguise and beats the tar our of her.  Ranma shows up, congratulates the
now stunned Mousse, then leaves.

Variations on this could be that Nabiki devises this plan for some reason
or another.  Or, she could possibly pull the whole thing off without Ranma
totally knowing what is going on.  There could be several other variations
on this, but I'm too tired to think of them right now.  Anyone care to take
a stab at this?

Dark Nation