Yes, perhaps in this fanfiction Bella could be a new Avenger. Fanfictions often introduce new characters or give existing characters new roles within the established Avengers universe.
I'm not sure about a specific 'bella is a avenger fanfiction' off the top of my head. But you can try searching on fanfiction websites like Archive of Our Own. There are likely many stories with that concept, and you can filter by popularity, ratings, etc. to find a good one.
The appeal of 'bella is a avenger fanfiction' lies in the combination of two different and popular universes. Fans of both the Bella character (presumably from Twilight) and the Avengers get to see how these two worlds collide. It's like a new adventure that mixes the powers and personalities of the Avengers with the unique charm of Bella.
Yes, it might exist. In the world of fanfiction, there are countless stories created by fans. Since Bella Swan is a popular character, it's very likely that someone has written a fanfiction involving her and the Avengers in some way. There are many fanfiction platforms where enthusiasts share their creative works.
Maybe it could be about Bella Swan having superpowers and joining the Avengers. For example, she could have a power related to her ability to resist vampire powers in 'Twilight' and use it to help the Avengers in their battles.
In some fanfictions, the 7th Avenger could be a time - traveler from the future. This person has come back to prevent a major disaster that the Avengers couldn't handle without their help. They know all about the future events and use this knowledge to guide the team, but also have to be careful not to change the timeline too much.
One great Avenger crossover fanfiction is 'Avengers and the Justice League'. It's really exciting as it imagines what would happen if the Avengers from the Marvel universe met the powerful heroes of the DC Justice League. The story is filled with epic battles and interesting character interactions, like Iron Man and Batman sizing each other up, and Thor and Superman having a friendly competition of strength.
Tony Stark and Steve Rogers are a very popular pair. Their contrasting personalities, with Stark being the brash playboy inventor and Rogers the moral - centered soldier, make for great body - swap scenarios. Another popular pair is Thor and Loki. Their complex relationship as brothers adds a lot of depth to body - swap stories. And Hulk and Bruce Banner also feature often. The duality of the man and the monster in one body is an interesting concept to play with in a body - swap setting.