A cool plot could be that Spider - Man is mistaken for a new meta - human in the DC universe. Wonder Woman takes him under her wing to train him, but they soon discover that there are villains from both universes working together. Spider - Man and Wonder Woman then lead a team of heroes from both universes to fight against the combined 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.
Maybe in the fanfic, Spider - Man and Superman cross paths. Spider - Man is initially in awe of Superman's powers but then realizes that his own agility and web - slinging can be very useful. They have to work together to stop Lex Luthor who is creating a device that could disrupt the fabric of both universes.
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.
Perhaps in the fanfiction, Spider - Man's sense of humor clashes with the more serious tone of the DC heroes at first. But then they realize they can learn from each other. For example, Superman could teach Spider - Man how to better control his strength, while Spider - Man shows the DC heroes how to have a more light - hearted approach in the face of danger. And they all come together to fight a common enemy like Darkseid who has somehow managed to get his hands on some of Spider - Man's tech.
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.
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.
One idea could be that Spiderman gets transported to the DC universe due to a multiverse glitch. He teams up with Batman to stop a villain who has stolen technology from both universes. Another could be that Peter Parker goes to Metropolis for a science fair and accidentally gets involved in a battle between Superman and Lex Luthor, using his Spidey - sense and skills to help Superman.
Maybe in this fanfic, there's a multiverse crisis that causes parts of New York in the Marvel universe to merge with Gotham in the DC universe. Spiderman and Superman meet and initially have a bit of a misunderstanding due to their different ways of handling things. But then they realize they need to work together to separate the two cities again. Spiderman could use his webs to help Superman contain some of the chaos while Superman uses his strength to move large structures that are in the way.
A cool plot could involve Wonder Woman, Flash, and Aquaman. They find themselves in an alternate dimension where the rules of their powers are all mixed up. Flash might not be as fast, Aquaman can't communicate with sea creatures as well, and Wonder Woman's lasso doesn't work as expected. They have to figure out how to adapt and work together to get back to their own dimension while also facing the threats in this new world.
Spider - Man and Green Lantern would be really interesting. Spider - Man's quick thinking and acrobatic skills combined with Green Lantern's power of creating constructs could lead to some amazing fight scenes and problem - solving moments.
Definitely Spider - Man and Harley Quinn. Their personalities are so different. Spider - Man's quippy and responsible nature against Harley's chaotic and unpredictable self would make for some really fun and interesting interactions.
One idea could be Ahsoka getting transported to the DC universe and teaming up with Batman. She could use her combat skills and Force - sensitivity to help him fight against Gotham's villains. Another plot could involve her meeting Wonder Woman and learning about the different types of power and heroism in the DC world. Maybe she gets caught in the middle of a battle between the Justice League and their enemies and has to use her Jedi wisdom to broker peace.