You could also consider a plot where the Avengers have to deal with a new threat that was actually created as a result of their own actions during the 2012 battle. This could lead to some internal conflicts among the team as they grapple with the consequences of their previous victory.
One idea could be exploring what if Loki had an unexpected ally within the Avengers. Maybe a character who was initially on the heroes' side but got swayed by Loki's charm or promises.
Another plot idea is to focus on the aftermath of the Battle of New York from the perspective of an ordinary citizen who witnessed it all. How did it change their life? And how do they view the Avengers now? This can add a new dimension to the 'Avengers 2012 Fanfic'.