Others have already pointed out the most relevant issues, but I'll rehash a little and cover a couple new ones.
You want Fleur/Ranma? Great! Genma sold Ranma to the Delacours for two baguettes and a croissant while on his training trip.
You do need to account for the cold war.
More importantly, you need to account for the sociopolitical framework of entirely different nation-states. In Potter, the British are rendered impotent by JKR's fiat/the suborned/cowardly Prime Minister. Further, the British ministry tries (badly) to hide from Muggles, appears to hold Merlin in high regard, and Hogwards is proud of less than a thousand years of history. Chinese magical organizations are unlikely to consider a thousand years as much beyond "those young whippersnappers", and while I'd expect a greater degree of integration between magic and the government and the martial arts community, the magical community is likely pre-communism, which could be... interesting.
What does the Japanese government think? Do they care?
I would note very seriously that Fleur is half-Veela (apparently half-human). I see zero confirmation in canon that full Veelas can use wands, and zero confirmation that they can't. House-elves, goblins, humans, and various part-humans are the only ones I've seen confirmation on wand-use from.
Note that I don't recally any Chinese or Japanese involvement in the Quiddich World Cup.
Note also that involving Ranma in the Harry Potter main storyline isn't as bad as involving Muggle technology, but is very unlikely to leave the original in anything like the state it was before. See http://www.fanfiction.net/s/3281389/11/Honor_among_Thieves
for an example.
If you take the movies into account, it's incredibly unlikely that any Ranma series martial artist can be hit by any want-fired spell that travels at a range of over a few feet, unless they're unaware. Spells travel too slowly.
You do really need to determine where the story is going first, how it ends, and then check your starting point to see if the characters would actually get there on their own, given, say, Ranma's fight first, second, third, and so on attitude.
Critical note: so, what about Genma? He's not going to abandon Ranma, or the Tendo marriage pledge.




