This idea is for a supernatural fantasy/ghost story. Ranma has for several months (offscreen parts during the manga) been sneaking into the school after hours and playing with a ghost named Hanako. Hanako is a very common ghost story told in Japanese schools (several movies and the story is told in just about all the schools with kids really being to afraid to go to the bathroom to the point that schools have started upgrading the bathrooms to make them less scary). To call the ghost you go to the third bathroom stall and say "Hanako-san, Hanako-san let's play a game". The ghost then comes out and says "Yes. Do you want to play." Depending on the story sometimes she is nice and wants to play, some times she scares people, and sometimes she kills people. She usually, and for the fic idea would, have short hair and wears the color red.
Ranma has been keeping her company some nights and they play games like Chess, Monopoly, Trivia Pursuit, Life (very popular in Japan), cards, Go, Shogi, Renju, etc. The thing is Ranma left for awhile and neglected to tell her where he was going. She starts to worry about him and goes in search for him, by randomly popping out of toilets (for the purpose of this fic she is metaphysically bound to them - place bound spirits are common in ghost stories, particularly vengeful ones, generally bound to the place they died usually by strong emotional ties). When he finally does show up and summons her she is initially glad to see him but upon learning that he was not in danger, just forgot to tell her he'd be busy for a few weeks and was not able to visit her, she gets extremely angry and drags him (still alive) either to the Sanzu river (Buddhist river of the dead), Purgatory (Catholic waiting room to the afterlife), or to Yomi (Shinto afterlife) - the Japanese would not see a problem with all three existing congruently and I don't know what afterlife I'd choose were I to write this.
That is really all I have for this idea, besides after that event instead of occasionally meeting supernatural creatures/spirits he does so with a greater frequency than he did in the manga (though he did encounter quite a few in the manga). Some would be dangerous such as the Teke-Teke (ghosts that are missing the lower half of their bodies and chase people while carrying scissors or a sickle with the intent of chopping off humans lower half and making them into Teke-Teke). Some would be harmless though freaky looking such as man-faced dog (Dog with the face of a man often seen digging in garbage, sometimes chases cars, generally tells people who happen to see them to leave them alone). Some would be helpful like the Baku (a monster that eats nightmares).
The ghosts would also have common ghostly powers, though some would be dependent on type, such as the ability to enter dreams (like the astral form of an old man Ranma went on a date with in the manga), hitodama (balls of spiritual fire that most of the ghosts had like the lady ghost who wanted her panties stolen or the Bakeneko, ghost cat, in the manga had), possession, intangibility, ability to place curses, shapeshifting, etc.