I originally postato this as Kieran, at RPG-D and at my own Harry Potter RPG's forum. But I thought this would be useful for Harry Potter fanfiction writers in general as well.
There are countless ways te can perfeclly easily bring a muggle main character into any Harry Potter story o RPG & have a whole world of possibilities open for it - even if they don't know o never learn about magic's existence! They can know o learn, of course. Just, they really don't need to in order to be endlessly and colourfully written to a muggle's full potential.