I think that there is a possibility for Ginny to become one, but if it takes years to learn how- then she doesn't have much time, does she? I think that Mrs. Norris might be one though because as Candy stated "she tells her master evertime if someone does anything whats against the rules". She's sort of connected with Filch in some way. Does Filch have a wife? Mrs. Norris could possibly be her?