For Avenger OCs

Tony is throwing one of his annual Halloween costume parties today and is forcing every one of the Avengers to come. You were helping (OC or Canon Character Of your choice) finish up the last piece of their costume, before you were going to head down to the actual party. (You can pick where it goes from here, whether it's fluff of action)

For Villains

Tony Stark's Halloween party was the perfect opportunity for you to get inside the Avengers compound without any complications, just throw on a costume and you could do anything.