This is just something that's been gnawing at me for a while and I finally decided to try scrawling the basics of it here and seeing if something useable might be brewed from the sludge. The main characters are Shampoo and Ukyo, and it's essentially shaped into a three-way pairing (Ranma+Shampoo+Ukyo). It involves some "playing" with the rules of Joketsuzoku, but as we get none of them beyond the Kisses of Death & Marriage even in the manga (and I work from the anime canon), that isn't such a problem.
The rough beginning I have is Shampoo sitting on a rooftop after her shift is over, thinking to herself why it is that Ranma never seems to have any time for any girl but Akane. After all, she and Ukyo both treat Ranma far better (in their point of view- remember how Ranma & Akane tend to act towards each other while either of the other girls are around) than the youngest Tendo, so why is it that he drifts towards her instead of either Shampoo herself or Ukyo?
Then she has a brainstorm; it's because whenever she and Ukyo are in the same place, they inevitably start fighting over Ranma, which makes them look worse in Ranma's eyes. And while they're fighting, Akane basically creeps in and steals Ranma out from under their noses. Upon realising this, Shampoo comes to a decision: while it's really something she'd prefer to avoid, tis a far, far better thing to get half the prize than none at all. So, she sets off towards Ucchan's Okonomiyaki to have a powwow with Ranma's cute fiancee.
Ukyo, naturally, is sceptical of Shampoo's claims to want to talk... and dumbstruck by what Shampoo is telling her. Basically, it's a way to "settle" their quarrel by taking advantage of two Joketsuzoku laws: the first, the fact exceptional male warriors (like Ranma) are allowed to have multiple wives, the better to pass on their genes, and the second the fact that outsider women who impress a Chinese Amazon can, if they wish, be adopted into the tribe as official Joketsuzoku warriors. If Ukyo agrees to become a (Japanese) Amazon, she and Shampoo can share Ranma by law.
That's about all I have for it this far. So, any comments?