Just a couple of ideas-

1) Genma and Soun finally get Ranma and Akane to marry. Two to five years
later Ranma realized that he let himself be pushed into an unwanted marrage
and that he really loves Ukyo (or one of the others). This could also work in
reverse all of Genma's and Soun's pushing causes Ranma and Akane to never
admit their true feelings and they later realizes that they were in love. This
would probably make a good one shoter, but I'm not going to write it.

2) I hate to bring this up with the new character debate going on but here it
goes:
Normal day in Nerima, until Ranma notices a girl that he either has never seen
before or just never paid any attension to. This would definitely be multiple
parts with the first part just having Ranma and the "new" girl (who will have
no martial arts skills, crazy ancestors, magical weapons, curses, etc.)
literaly run into each other. They could developse a crush on each other and
eventually start trying to see each other behind the backs of the other
fiances. This will be my first fic if I don't hear that it's been done. I got
this idea when I was thinking how unfair it is that Ranma always ends up with
one of his fiances, whom (please no flames/lengthy discusions this is just my
opinion) Ranma really shows very little or no interst to. This would take
place before the end of the manga for the abvious reason of Ranma and Akane
falling in love (please no flames/lengthy discusions on this opinion either).