Imagine that after the war the Marauders were reunited, James, Sirius, and Remus and that they return as ghosts to Hogwarts. Then one day Minerva is walking down the corridor when she hears a loud bang coming from one of the classrooms and her immediate reaction is to shout, 'PEEVES!' and she storms into the classroom and see the Marauders and is like, 'not you boys again' and she's ready to tell them off when she sees Fred Weasley and she just stops and smiles, cause they've adopted Fred as the fourth Marauder and she's just so happy because they are.