You were just an ordinary person, unaware of the existence of magic. Then, one day, something happens. Maybe that old wand you inherited turns out to be more than just a decoration, or there's an emergency and you're the closest compatible person. However you caught their attention, you are approached by the Puuchuu, a group of magical beings dedicated to raising up magical girls to fight the forces of Chaos. They want to make you a magical girl.
This would be an RP based on the Accidental Magical Girl CYOA with its Patron Module expansion, found here:
Core
Patron
It'll be a bit different than a lot of RPs in that the characters will be, for the most part, random. There are options (Smug, the coins) that make things less random, but they come at the cost of making your character weaker overall. I think it makes for an interesting RP challenge. I'd prefer for the characters' personalities to be based on the powerset you roll, but there could be intersting implications in somebody whose powers are at a contrast to their disposition.
As for the setting, I was thinking it would take place in a fictional city in America, but still part of the wider AMGCYOA universe described in the CYOA itself. We could figure that out further depending on what everyone thinks, though.