Temple of Ranma's Sailor Senshi Seifuku

It sounds like you are looking for Potter the Stone, by CRose.

Heh. I think that was the first Ranma fic I read at ff.net. In fact, it was that fic who made me check the Ranma section and got me addicted.

I agree both that it was quite a good read and that I would like to see it continued. There are so many plot elements left to explore, such as

SPOILER - Show Spoiler

Ranma's newly developed magical core, how Harry's ki will mix with his developing magic, how Sirius will handle having his godson raised by someone like Ranma, how Ranma and Sirius would interact, and far more.

It also gives a lot of room to introduce new elements into the story. If Ranma were to spread what he knew at the end of the story, you could have an influx of martial artists to London trying to learn the same things, especially young martial artists who might start challenging Harry and declaring themselves to be his fiancee, whether he likes it or not...

Being the most dangerous fighter in the world didn't save him from constantly having problems in Nerima. If anything, it compounded them because everyone wanted to test themselves against 'the best.' Serious physical opposition would be difficult to come up with for Ranma, but not so hard for Harry. Say the basilisk escapes somehow and manages to petrify Ranma. Harry would then be the one dealing with the basilisk with admittedly greater skills, but without the loyalty to Dumbledore that allowed him to summon Fawkes, so no goblin-forged sword to kill the basilisk, and his training would make 'white belt luck*' far less likely.

Also, as a student advance it becomes common to have them face any challengers before the challenger has the right to challenge the master, so Harry could have plenty of issues with all sorts of martial arts challenges. The legal challenges are another window to open, especially if you rewrite the closing deus ex machina wrap up. The difficulties involved in Ranma getting into magical duels, where he still has less power than Harry, against practitioners of obscure magical arts could also be an interesting way of working more Ranma style problems into the story. Ranma is always willing to play by the rules of the person challenging him(within certain limits) so that he can prove that he is better at their style than they are.

*The phenomenon of someone who has no clue what they are doing sometimes winning a match against someone much more skilled simply by doing something so stupid and unpredictable that their opponent cannot adjust to it in time.