Heavenly Ranma
Disclaimer: As always, nothing involved in this is my property nor is it used for profit. I'm merely postulating an idea that I think would be interesting.
General idea: Years ago a great battle was fought for a cursed sword. The wielder fell, the sword implanted in the ground. But, like all good epic battles, it was raining, unlike the game, when Nariko falls to the influence of the sword, her cry for vengeance resonates with the blade, cleaving the plane in twine and demolishing the approaching army entirely. The Valley of Jusenkyo is born, the powers of the cursed blade leaking into the water from the riddled ground. Nariko's soulless, but technically still alive body drowns in the pool of water gathering around the sword. The power slowly leeches off to maintain the pools and the sword is powerful enough to even maintain a spring of drowned Ashura, it's death magic siphoning to create the cursed training ground.
Years later and no one has fallen in the Spring. In fact, though it is near the center of the grounds, no one knows what spring it is as it won't permit any near it. The sword was semi-sentiment after all. Nodoka was the descendent of the last of the clansmen charged with defending the sword, so when a member of the bloodline entered Jusenkyo, Ranma is naturally drawn towards that spring rather than drowned girl. When Ranma falls in the Spring, one hand wraps around a handle and Ranma pulls the sword out, causing the spring to drain. Nariko's stuff is all preserved, Ranma will find it when she returns to the spring after the chase... Perhaps Nodoka has Kai's Gatling-bow.
A vibrant red-head bursts from the spring with the gear and runes branded into her right arm. In her incoherent rage at her father, she doesn't notice much else aside from the swell of her chest, the shift in her balance and the lack of a special something between her legs. Cue the usual chase sequence against the father, Ryoga gets his usual curse but Ranma instinctively uses the sword against Genma. Ranma, when calm, gathers the gear and reflexively changes into the outfit and places the heavenly sword in the sheath at her lower back. She also has the longsword which she wears at her hip. The guide finds that hotter than normal water will change Ranma back, but slower than usual, while she will shift female at a higher temp than normal faster than usual.
Goes to Amazon town, usual scene with Shampoo up until the kiss where Ranma prevents Genma from running off and calls into question the honor of the Amazons very vocally. Where Ranma had a variable honor (strong, but variable) from his time raised by Genma, some bleed over of Nariko's personality happened and she is deeply offended by the casual nature the Amazons treat life with. Shampoo and the Amazons press the attack, and she demolishes them without killing them, by using the Heavenly Range sword. After the fact, she passes out and her father drags her off. I think the Amazons will show up in Nerima, but Shampoo will have been in no shape to chase Ranma across China so that didn't happen.
Ranma will get some bleed through of personality which means he will gain an incredibly honed sense of honor though it will *not* follow traditional Japanese lines nor will he seek a cure after a while. Initially he will want one, but he won't eventually as bearing the sword becomes a very large honor debt, in fact it becomes the most important facet of his/her life.
Ranma will train with the sword, not even realizing its supposed curse, which has actually bled out into Jusenkyo. It is still a Heavenly Blade, but it will no longer kill the wielder. It, however, will remain insanely powerful and Ranma will eventually figure out how to shift the blade between its 3 forms, he will naturally favor speed and ranged over power. Incidentally, while the sword does not kill him, it still drains some life force at a much slower rate than it used to. So, until his reserves are built sufficiently (say, the Moko Takabishi arc) Ranma will pass out if he/she uses the sword too much.
Things will be different with certain situations, for example, Ranma will deal with Ryoga and his curse with brutal honesty and efficiency. Kuno will receive a world class beat down for questioning Ranma's honor, etc.
I'm unsure how to proceed at this point, as I feel that Nariko/Ranma would be far more efficient at dealing with the stresses in his/her life and the Heavenly Sword would easily allow Ranma to overpower his usual opponents, even factoring in a basal ki-leeching effect. I suppose Ranma wouldn't use the sword often, preferring to rely on the long sword instead. At this point, there would have to be some crossover elements to another series, as Heavenly Sword is set too far in the past to use it, and the sword is designed to enable one person to fight entire armies. Something like an apocalyptic invasion of Sailor Moon-esque foes could perhaps work, but I haven't thought that far ahead.
I probably won't ever get pick this up, but I started playing through Heavenly Sword and I think it would be a really cool idea to cross Ranma with it. And if anyone wants to attempt to write something along these lines please let me know as I'd love to read it.