One interesting concept could be alternate versions of Hogwarts. For example, a Hogwarts where the houses have different characteristics or where the magic taught is more focused on a particular type of magic like dark magic being taught openly (in a more morally grey fanfic). Another concept might be different fates for the main characters. Harry could be a Slytherin instead of a Gryffindor, which would lead to a completely different story arc. There could also be crossovers with other fictional universes in the multiversal fanfiction, like Harry Potter characters interacting with characters from Lord of the Rings or Star Wars.
It expands the story by introducing countless alternate realities. For instance, we might see a world where Voldemort won and how the remaining characters survive and resist under his rule. This creates a whole new set of storylines and character arcs. It also allows for the exploration of different character relationships. Characters who were enemies in the original might be friends in a multiversal fanfic, like Harry and Draco.
The Avengers originated in Marvel Comics. Marvel has developed a vast universe of characters and storylines, and the Avengers are a key part of it. Their adventures have been adapted into various forms of media, including movies and TV shows.
Captain Marvel is Carol Danvers. She gained her powers through an interaction with alien technology and has become a powerful hero in the Marvel universe.
Marvel justifies Marvel Comics 1000 by presenting it as a celebration of their rich history and a platform to introduce fresh and exciting story arcs. It could also be a strategic move to attract new readers and reignite the passion of long-time fans.