Definitely Iron Man. Their relationship in the official Marvel movies has already been very popular, so in fanfiction, writers love to explore more of their interactions. Iron Man can provide high - tech support to Spider - Man, and they can have some really smart and witty conversations.
Venom is also a very popular character to cross over with Spider - Man. The complex relationship between Peter Parker and the symbiote is always fascinating to explore. It can be a battle of wills, or they could somehow find a way to coexist in some unique fanfiction scenarios.
Fiona is also a top choice. Her relationship with Shreck in the movies gives a lot of material for crossovers. Fanfiction writers might explore how she and Shreck interact in different universes or scenarios, like in a world where they are not in the same fairytale but a different, more modern world.
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.
Mary Jane is also very popular. Fans love to write about her relationship with Peter, whether it's the ups and downs or how she deals with Peter's double life. In some fanfictions, she might even have a more active role, like helping Spider Man in some way.
Ben 10 is a favorite for many. As I said before, the clash between Johnny's human - centered confidence and Ben's alien - powered abilities is exciting. Also, characters like Dexter from Dexter's Laboratory are often used. The scientific genius of Dexter and the not - so - smart Johnny can lead to hilarious and complex stories. Courage the Cowardly Dog is also a great choice as the spooky and strange world Courage lives in can be a great backdrop for Johnny's adventures.
Thor is often crossed over with the Hulk. They already have a history in the official comics with their epic battles. In fanfiction, it could be about them teaming up more often or having to face a threat that only their combined powers can defeat, like a powerful cosmic entity.
Sherlock Holmes is also quite popular. Holmes' deductive skills against Darth Vader's power in the Force would make for an exciting story. Holmes could be trying to figure out Vader's motives and secrets, while Vader might see Holmes as a nuisance or a potential ally depending on the situation.
Definitely Bell Cranel. If he is crossed over into the Naruto world, his growth as an adventurer in Danmachi can be compared to the growth of a ninja in Naruto. His determination to become stronger is similar to Naruto's. Also, Ais Wallenstein is popular. Her combat skills and beauty would make for an interesting combination in the Naruto universe, perhaps having epic battles with Naruto's strong female characters like Tsunade.
Obviously, Spider - Man and Iron Man themselves are extremely popular. They are the main draws as they are the two iconic heroes. But characters like Pepper Potts might also be popular as she could be involved in stories related to Iron Man's actions in the fanfiction.
Probably Naruto himself would be a very popular character to cross over. His energetic and determined personality could bring a lot of action and drive to a crossover story. In Vampire Academy, Rose Hathaway could be another popular choice. She is strong - willed and a skilled fighter. When these two characters meet, there could be a great dynamic of them either competing or teaming up. Another popular character from Naruto that could be used is Sasuke. His brooding and powerful nature would contrast well with the vampires in Vampire Academy.
In Naruto - Scott Pilgrim fanfiction, Rock Lee is a character that many fans like to cross over. His unique fighting style based on pure hard work and determination would be really cool to see in the Scott Pilgrim universe. And from Scott Pilgrim's side, Wallace Wells could be a popular crossover character. His laid - back and sarcastic personality could create some humorous moments when interacting with the Naruto characters.