A great crossover idea could be a team - up between Spider - Man and Deku. They could learn from each other's sense of responsibility. Spider - Man has been dealing with the weight of his powers for a long time, and Deku is constantly growing into his role as a hero. They could share their experiences and fight a common villain that threatens both their worlds.
His spider - sense would be a very useful power in the MHA universe. Given that many battles in MHA involve sudden attacks and surprises, Spider - Man's ability to sense danger before it happens would give him an edge. He could use this to warn his MHA allies and prevent ambushes. Also, his super strength would make him a formidable force against the powerful villains in the MHA world. He could hold his own against even the strongest quirks, like All - Might's strength - based quirk.
One interesting crossover could be having Midoriya from MHA gain Spider - Man's powers. He could use his One For All quirk in combination with the web - slinging ability. Picture him swinging between the skyscrapers of Musutafu while also using his super strength in combat. Another idea is Peter Parker attending UA High. He'd bring his scientific knowledge and web - shooting skills, and would probably befriend characters like Izuku and Bakugo. He could also learn some new combat moves from the hero students there.
The 'team - up' trope is always popular. Whether it's Spider - Man teaming up with Deku and his friends to take on a powerful villain, or the MHA heroes joining Spider - Man to fight a threat in New York. Also, the 'mentor - mentee' trope can work well. Spider - Man could mentor a young hero in MHA, teaching them about responsibility and using their powers effectively.
A possible idea for a MHA RWBY crossover fanfic is to explore the idea of semblance and Quirk evolution. The characters from both shows could train together and discover new ways to enhance their abilities. For instance, Izuku Midoriya could learn from Ruby about using her Crescent Rose in combat and in turn, show her how to better control her power flow. The cultural differences between the two worlds could also add depth to the story, like how the heroes in MHA are more publicly recognized compared to RWBY, and how the characters react to this disparity.
One plot idea could be that Spider Man and Spider Girl team up to face a new super - villain who has the ability to manipulate time. This would create a lot of interesting scenarios where they have to outwit the villain while also dealing with the chaos of time being altered. Another idea is a story where Spider Girl is from an alternate universe and she accidentally ends up in Spider Man's universe. They initially have some misunderstandings but then realize they can work together to protect both universes from a common threat.
One idea could be that Spider - Man gets accidentally transported to the DC universe. He teams up with Batman to take on a common enemy like the Joker who has somehow gotten his hands on some Oscorp tech. They combine their detective skills and combat abilities to stop the chaos.
In an 'ultimate spider man crossover fanfic', a really interesting crossover could be with Venom. But instead of being enemies, they team up. Maybe there's a new symbiote - like threat that is even more powerful and they have to combine their powers to defeat it. Another idea is a crossover with the Guardians of the Galaxy. Spider - Man gets accidentally transported into space and has to team up with Star - Lord and the gang to get back home while also fighting off some space - pirates.
One idea could be that Spider - Man gets accidentally transported to the DC universe. He teams up with Batman to take on a new villain that is a combination of their rogues' galleries. For example, a villain who has the powers of Electro and can also control minds like Brainiac.
Maybe there's a multiverse - related plot where the universes of Spider - Man and Young Justice start to merge. This causes chaos as characters from both worlds start to appear in the wrong places. The Young Justice team and Spider - Man have to work together to find a way to separate the universes again. They'd face challenges like dealing with different power levels and the cultural differences between their two groups.
Another possibility is a crossover with the world of Naruto. The characters from MHA could participate in the Chunin Exams. Bakugo might be really into showing off his skills in this new and different kind of competition.