Well, in a Ben 10 and Marvel Zombies fanfic, a great crossover would involve Ben's Omnitrix getting corrupted by the zombie virus. This could lead to some of his alien forms having zombie - like traits. Also, imagine Ben teaming up with the few remaining non - zombified Marvel heroes like Spider - Man. They could use Ben's shape - shifting abilities and Spider - Man's agility to outwit the zombies.
Well, a great crossover idea in naruto and marvel fanfic is the Avengers team coming to the Naruto world. Imagine Iron Man trying to figure out the chakra system while Captain America is learning ninja techniques. This would bring a whole new dynamic to both universes as they interact and learn from each other.
One interesting crossover could be the use of Ben's Omnitrix to transform into a species similar to a Star Wars alien. For example, he could transform into something like a Wookiee - like creature and interact with the Star Wars characters in unique ways. Another aspect could be the combination of the Force from Star Wars and Ben's alien powers. Maybe a character with the Force tries to steal the Omnitrix, leading to an epic battle.
One idea could be to explore the origin of the zombie virus in the Marvel universe more deeply. Maybe it was a failed experiment by a mad scientist in Stark Industries. Another idea is to focus on a group of survivors, like Spider - Man and Daredevil, trying to find a cure while fighting off zombified heroes. You could also write about how the zombified heroes' powers are affected by their undead state.
One interesting crossover could be Master Chief meeting Iron Man. They are both iconic in their own universes. Master Chief with his Spartan strength and combat skills, and Iron Man with his high - tech suits. They could team up to face a common enemy that threatens both the Halo and Marvel universes, like an interdimensional threat that is after the advanced technology in both worlds.
The story could start with Ben arriving in the Marvel universe just as the zombie outbreak begins. He'd be confused at first but quickly realize the danger. He might start by using some of his basic alien forms to test the zombies' weaknesses. As the story progresses, he could meet some of the Marvel heroes who are still alive and form an alliance to find a cure.
One interesting crossover could be Ben using his alien forms to help the Avengers in combat. For example, Four Arms could provide extra strength in a fight against a powerful enemy like Thanos. Another could be Heatblast teaming up with the Human Torch to create an even more intense heat-based attack.
There might be a 'Ben Ten Marvel Fanfic' where Ben meets the X - Men. Imagine Ben showing off his alien forms to the mutants. He could learn from Professor Xavier about controlling his powers better, while also teaching the X - Men some new combat techniques using his alien abilities. For example, Heatblast could help Iceman with his control over heat and cold in a new way.
Another interesting plot could be that some of Ben's alien forms are mistaken for new mutants in the Marvel Universe. He then has to prove that he's not a threat while also helping the X - Men deal with a common enemy that is interested in the genetic makeup of both mutants and Ben's alien forms.
One interesting crossover could be the Quirks from 'My Hero Academia' interacting with the superpowers in the Marvel Universe. For example, imagine Deku's One For All Quirk combined with the powers of Spider - Man. He could swing from building to building with his web - like attacks while still having the raw strength of One For All.
One idea could be a crossover with the Powerpuff Girls. Ben could team up with the girls to fight a common enemy that is causing chaos in both their universes. Another could be a crossover with the Teen Titans. Ben's alien forms and the Titans' powers would make for some epic battles and cool team - up moments.