Well, I assume we're still talking about HP whatever we're doing.
So we could keep it Hogwarts/modern, just start from scratch. I think a lot of people are fond of this because it is relatively easy to relate to the world. There's still some variation, though. I think it might actually be best to start in a later year(I'm fond of fifth) because, let's face it, none of us are eleven(most of us probably aren't fifteen either, but it's easier, I think). It lets us explore more mature themes, we can have assumed rapports and relationships, as well as pre-acquired magical skills, so we don't have to spend so much time on exposition/learning curve. Let's be honest, everyone wants to get to the magic. And I think people can get frustrated when, for the first ten pages, every other interaction is "Oh, hi, we haven't met, my name is Barney Rubble. You're house Gunderstank? No way, Weaselthorp for the win!"
Overarching plot and conflict can vary. I've got some old ones I can whip out, including other mysterious villains, or Dark Art clubs(kind of the Anti-Dumbledore's Army). The latter would be very character driven.
We could also do stuff outside of school. If you remember in book seven it's briefly mentioned that Hogwarts grads usually do a tour around the world to learn about other magical cultures, could be a small group RP focusing on a group taking their traditional pilgrimage.
Could be Aurors vs Dark Wizards.
Could go medieval Hogwarts, or medieval in other countries. Depends on what people like.
@Eyeris@MiddleEarthRoze Don't know if either of you want to chime in/get in on this.