Maybe a little late to the game, but I mod a future-gen HP game and we've just opened a plot arc that might work for you. The game is full_circle, plot-centric, slightly dark, mature, fast-paced game. We're OC friendly and generally pretty friendly all around.
As far as growing into the setting, our newest plot arc involves Muggleborn students who didn't receive letters to Hogwarts and thus weren't alerted to why they could do the strange things they could do.
So, you'd certainly be welcome to app in a Muggleborn who can be discovered in-game and either tutored or eased into school depending on the age, so you can learn as your character does. The only restriction we'd have to place, given the arc of the plot, is that your pup couldn't be any older than 20 if you opted to go this way with it.
Of course, you're certainly welcome to fill any other role and we're always happy to answer questions.