Well, a cool Ultra Man crossover fanfiction could involve Ultra Man crossing over with the Avengers. Imagine Ultra Man showing up in the Marvel Universe and having to deal with villains like Thanos. He could use his unique powers to assist the Avengers in their battles. Also, a crossover with the Justice League would be interesting. Ultra Man could bring his own brand of heroism to the DC Universe and collaborate with Superman, Batman, and others to protect the world. A crossover with Pokémon is also a possibility. Ultra Man could help Pokémon trainers in a world where strange and powerful creatures are in danger from an unknown menace.
One idea could be an Ultra Instinct Goku crossing over with the Dragon Ball Super universe and the Naruto universe. He could end up in Konoha and use his Ultra Instinct to sense and counter the attacks of the ninja there. Maybe he'd even have to fight some of the powerful ninjas like Naruto or Sasuke in a friendly sparring match to test their skills against his new form.
Maybe Goku gets mixed up in the events of the Injustice storyline. He could initially be confused by the alternate version of Superman who has gone bad. Goku, with his pure heart and Ultra Instinct, might try to bring the corrupted Superman back to the light. This could lead to some intense battles with the Injustice League as Goku tries to set things right.
One idea could be a He - Man crossover with Thundercats. Imagine He - Man and Lion - O teaming up to fight a common enemy that threatens both Eternia and Thundercats' planet. Another could be a crossover with the Teenage Mutant Ninja Turtles. He - Man could visit the sewers of New York and teach the turtles some of his combat moves. A third idea is a crossover with Ghostbusters. He - Man helps the Ghostbusters to capture some powerful spectral entities that have somehow made their way to Eternia.
One idea could be a crossover with the X - Men. Spider - Man could find himself in the middle of a battle between the X - Men and some common enemy, like the Sinister Six. He'd have to figure out how to work with mutants, which is a whole new experience for him. Another idea is a crossover with the Fantastic Four. Maybe Spider - Man gets caught up in one of their space - related adventures. Reed Richards could be fascinated by his web - shooters and try to study them, while Spider - Man would be in awe of their super - scientific gadgets.
A crossover with 'Dragon Ball' would be cool. Picture Saitama facing off against Goku. It would be an interesting contrast between Goku's love for a good fight and Saitama's boredom with opponents who can't challenge him.
A crossover with Fullmetal Alchemist could also be fascinating. Picture Edward Elric and the D Gray - Man characters collaborating to understand the mysteries of the Gate and the Innocence. Maybe they find out that there are connections between the two in terms of the source of power and the balance of the world. They could also face off against enemies that are a combination of Homunculi and Akuma.
Another interesting plot could be that a mad scientist creates a device that can separate Venom from his host. However, the separation goes wrong and the symbiote energy starts to spread across the city, infecting people. Spider - Man and Venom must work together to find a way to contain the energy and save the city.
A crossover with the Avengers is also great. Picture Spider - Man having to prove himself to the more established heroes like Iron Man and Captain America. He could bring his unique street - level hero skills to a big - scale Avengers mission.
Well, a cool Venom and Spider - Man crossover fanfiction idea might be that there's a strange alien invasion. But this time, the symbiotes are being controlled by an even greater evil. Venom, having his own sense of justice, decides to work with Spider - Man. They start off not trusting each other much, but as they face the challenges, they learn to rely on one another. Also, perhaps Spider - Man has to deal with some of Venom's more violent tendencies during the fight.
How about a crossover with 'Attack on Titan'? Saitama could find himself in the world of the Titans. His ability to defeat any enemy with one punch would be extremely useful against the Titans. He might not understand the complex politics and fear that surrounds the people in that world at first. But as he starts taking down Titans left and right, he would become a sort of legend among the survivors.