This fan fic idea is set the day after the failed wedding in the manga. Ranma and Happosai have fought a lot in the manga and Ranma is pretty upset with Happosai for drinking the Nannichuan; though not a lot because he assumes it was an accident. Even though Ranma is not that upset he is not above getting some petty revenge against Happosai for drinking it. To do so Ranma searches Happosai's room (wouldn't be the first time, Ranma searches peoples rooms pretty frequently in the manga), looking for something to use against Happosai (incidentally stealing any magic item/ martial arts books/scrolls/pressure point charts that Happosai happened to leave there). In his searching of Happosai's room for something to use against Happosai, Ranma finds a barely legible notebook that Happosai has written. It turns out to be a diary (Happosai got distracted and forgot to hide it away properly). Ranma reads through it hoping to find something to use against Happosai but it mostly lists girls and their three sizes and critiques on various under garments brands/styles, but interspersed amongst that is some useful information. One of the things, and the most important for this story idea, that Ranma finds out is that Happosai knew that it was not sake and that Happosai has acquired Nyannichuan to recurse Ranma if Ranma did find a cure.
Needless to say that really pisses off Ranma. Prior to finding Happosai's diary Ranma was considering just destroying Happosai's collection of undergarments and helping the girls at school booby trap the girls locker room if he didn't find anything to use against Happosai in Happosai's room, after finding it he feels like that would not be sufficient revenge. A problem Ranma has is that he doesn't know what to do for revenge, he does not want to kill or seriously hurt Happosai but does want to do something worse than just antagonize and inconvenience him.
Ranma being the impatient live in the moment type person he is generally would come up with a plan and implement it often times without thinking it through all the way. This time because Ranma is pissed and wants to come up with a really good revenge spends days trying to come up with a good plan. He comes up with hundreds but discounts them; some he feels are too excessive (those that would end up with Happosai dead), others are discounted because they are insufficient, still others are discounted because he actually spent enough time to notice the flaws in his plan, etc.
That is where the first major problem I encounter with this fic idea comes about. I can think of many possible ideas for revenge but have yet to think about a really good one.
Lets skip ahead a bit to after Ranma has successfully come up with a good revenge plan. He implements it and while it does not go exactly as planned it still is successful. This of course brings the fic to the point where Happosai retaliates. Happosai's retaliation is simple, he does what he did before ordering things in Ranma's name, trying to get Ranma blamed for things, pokes holes in the walls, etc. That does not work so well, it is sufficient to get Ranma kicked out of the Tendo house for a bit but that does not bother Ranma very much (he fully expected and planned for worse than that). Happosai, not having any magical item available to use on Ranma (Ranma stole them earlier) decides to go to various shrines and free trapped spirits - things like Kappa, Oni, and Tengu just causing as much trouble as he can knowing Ranma would go and trap them.
Ranma is then forced to hunt and capture these things, learning more about spirits and things like spirit wards. This is where there is a twist in the story. I'm not sure if I want to give it away at this time because I may, if not soon, eventually decide to write this story.