One common theme is power struggle. Since all three characters - Harry Potter, Severus Snape and Lord Voldemort - are associated with different levels of power in the original series. In fanfiction, this power struggle can be explored in new ways, like in a different magical setting or with altered character abilities.
The X-Men are part of the Marvel Comics lineup. They have their own unique place within the Marvel multiverse, with a dedicated fan base and a significant impact on the comic book world.
X-Men is not associated with DC Comics. It's a Marvel creation. DC and Marvel have their own unique superheroes and comic book worlds that are independent of each other.
One unique feature is the combination of magic systems. In 'hp x got fanfic', you can see how the spells and potions from Harry Potter interact with the more natural and mysterious magic in Game of Thrones. For instance, a wizard might try to use a healing spell on a character injured in a battle in Westeros.
One common theme is power comparison. Since both series have their own magic systems, fanfics often explore how the powers in khr stack up against those in hp. For instance, the flames in khr vs the spells in hp.