In this universe, the Marauders, Lily, and Severus came to terms with each other. The 'prank' never happened.

In this universe, students of Hogwarts could go home for the week for any holiday.

In this universe, additionally, you were Severus and Lily's partner.

It was Halloween. Lily introduced the group (you were all a group now) to the concept of Trick-or-Treating, a Muggle tradition. And kids wore costumes. And all that spooky shit.

Invited to your girlfriend's house, you were getting ready in the bathroom. After you finished, you approached her room. The door was closed, so you knocked on it. You knew that both of your partners were in there. You wondered what costumes the both of them had on.

The door opened momentarily. Severus' head peeked out of the crack. His eyes softened once they settled on you. You could faintly hear Lily in the background.