The Scarlet Witch could be a key character. Since she was central to the House of M event in the comics, her presence could add a lot of depth. Maybe she has some sort of connection to Spider - Man in this fanfiction, like she sees him as a key to either maintaining or breaking the false reality.
Quicksilver could also be introduced. He's Scarlet Witch's brother and his super - speed could be used in interesting ways. He could be on a mission to either help or hinder Spider - Man. For example, if he believes that the House of M world is better for mutants, he might try to stop Spider - Man from trying to change things back.
A new character could be an alien who crash - lands on Earth and has a connection to the technology that the Agents of Shield are researching. Spider - Man would be intrigued by this alien's abilities and the Agents of Shield would want to study it. This could lead to a story where they have to protect the alien from other forces who want to use it for their own evil purposes.
Penni Parker could be a great focus. In the movie, she doesn't get as much screen time as some of the others. Fanfiction could explore her backstory more, like how she became the SP//dr in her universe and what her daily life is like when she's not in the multiverse - saving action.
One idea could be that Spider - Man discovers a hidden power within the House of M world that allows him to break the false reality. He could start seeing glitches in the altered world order and use his wits and newfound power to set things right. Maybe he teams up with some other characters who also sense that something is wrong.
A young street - smart kid from the Vegas slums who befriends Spider - Man and helps him navigate the seedy parts of the city that he's not familiar with. This kid could have dreams of leaving the slums and Spider - Man becomes an inspiration for him.
One character that could be added is a new love interest for Peter Parker. Maybe a scientist who works at Oscorp and has her own set of skills and secrets. Another could be a young mutant who looks up to Spider Man and wants to be his sidekick. This would add a new dynamic to the story as Spider Man has to train and protect the young mutant.
One underrated character is Peni Parker. In fanfiction, you could explore her relationship with her SP//dr suit in more depth. Maybe she has to deal with some glitches in the suit that are affecting her performance as a Spider - Hero. Another character could be Spider - Ham. You could write a story where he has to use his unique animal - based abilities to solve a mystery in the multiverse that the other Spider - People can't.
One underrated character is Robbie Robertson. He could be given a more central role in a fanfiction. Maybe he discovers some shady business going on in the city that is related to the villains Spider - Man faces. He could then team up with Spider - Man to expose the truth. Another underrated character is Betty Brant. She could be developed more as a strong - willed journalist who is trying to break big stories, and her relationship with Peter Parker could be explored in a different way.
Black Cat could be a great partner. She has her own set of skills like acrobatics and thieving abilities which could be used in a different way to fight crime. Her complex relationship with Spider - Man also adds an interesting dynamic to the story.
Venom is quite popular in Spider - Man fanfiction. His complex relationship with Spider - Man can be explored in many ways. He can be a villain, an anti - hero, or even an ally in some really creative fanfictions. Another popular one is Gwen Stacy. Fans love to write about her and Peter's relationship, especially in alternate timelines.
One underrated character is Robbie Robertson. He could be featured more in fanfiction. He has a connection to the Daily Bugle and could be involved in a story where he discovers some secret about Spider - Man and has to decide whether to expose it or help protect him. It could add an interesting layer of moral dilemma.