So...
MC is reborn/born in the Harry Potter universe
Make the MC the same age as Harry Potter.
When the MC was a child his home town got attacked by Werewolfs and he ended up being bit which lead him to becoming one.
Write about the difficulties of living as a werewolf and how people react to finding MC is one.
Write about his experiences in Hogwarts, how he needs to hide once a month without anyone noticing.
Maybe you can have MC be able to transform at will or when he transforms he doesn't lose his rationality.
Troubles with the ministry and how he deals with them. You can even make the MC the heir of a good house.