I've been trying to find a certain fanfic for awhile. What I remember of the begining plot is Ranma and Saffron tied in their fight and both died. Saffron was reborn and, with his adult mindset and a clear mind, came to Ranma's funeral to apologise. When they try to cremate his body, magic acts up and he turns into an icy egg. Nodoka takes it home, puts it in the freezer or something, and it eventually hatches into a baby Ranko, who has magic powers over water and ice, including the ability to manifest wings made of water. Ranko rapidly ages and during a picnic they meet the outers with a baby Hotaru, who becomes best friends with Ranko as they age at the same rate.
One more detail that might help, I'm pretty sure that the author mentioned that it would eventually be a crossover with Harry Potter, but I can't be 100% about that since it's been so long since I read it.