The fanfiction could also delve into the backstory of how they are cousins. Maybe there was some long - lost family connection that was only recently discovered. This could open up a whole new plotline about hidden family secrets and a shared heritage. And of course, there would be the potential for some epic team - up battles against powerful supernatural enemies, with Harry's magic and Sam and Dean's hunting skills combining to save the day.
In this type of fanfiction, it could be really interesting. Harry might bring his own set of skills and a different perspective into Sam and Dean's world. Maybe he'd introduce some unique magic that could help them in their hunts. For example, he could use spells to track down monsters more easily or protect them from certain types of dark magic that they haven't encountered before. It would be a cool crossover, mixing the world of Harry Potter with the Supernatural universe.