There's "Nova" by skyrover7 where Ryoko is Ranma's mother, and she's Nodoka also
http://www.fanfiction.net/s/2944539/1/Nova
I also remember one where Ranma fell into the Spring of Drowned Mare and was never hit with hot water and had a child and became a legendary race horse...
It seems like an excellent idea. That is definitely one story I would enjoy reading.