A Marauders story concept, and hear me out here -
A journal of Peter Pettigrew written in during his time in Hogwarts and during the First Wizarding War. The journal tells stories of his good times with the Marauders, but also the times where he felt outcasted or betrayed.
After all, every hero has a reason for turning evil. And Peter Pettigrew has many.
(Side note - has anybody done this yet? I have been pondering making this for YEARS and would be genuinely surprised if it hasn't been done during my procrastinating time).