A rather simple story idea I came up with, that I am considering writing, is a collection of short stories featuring the nightmares of the characters. This idea came from the fact that we see several of the characters nightmares in the manga (most often Ranma's though - he has a lot). The basic structure of the fic would most probably be one nightmare per chapter.
Thinking of nightmares for the characters is pretty easy. Take Ranma for example; Could have nightmares about the Neko-ken, being trapped as a girl and losing his identity, having Tatewaki's children, the battle with Saffron with different outcomes, everyone wanting a piece of him and tearing him apart, losing his strength forever and having to rely on others, his mother finding him to be unmanly, etc.
The other characters, like say Akane, are almost as easy to come up with nightmares they might have. Her losing her mother, the pervert boys who attacked her every morning winning, ghosts, the vengeance doll retaining her body leaving her trapped in the doll, etc.