One idea could be that Ben Tennyson's Omnitrix gets mixed up with some of the Infinity Stones' energy. This causes his aliens to gain new powers related to the Stones. For example, Four Arms could have super strength on a whole new level like Hulk. Another plot could be that the heroes from both universes have to team up to fight a villain who is using technology from the Ben 10 world and magic from the Marvel universe. Maybe a new character who has stolen some of Doctor Doom's knowledge and some of Vilgax's power.
A cool plot could be that Ben accidentally travels to the Marvel universe and is mistaken for a new mutant. The X - Men initially think he's a threat but then realize he can be an ally. Ben then has to help them fight against a common enemy like Apocalypse who is interested in the power of the Omnitrix. Or perhaps Ben's presence in the Marvel universe starts to disrupt the balance of powers, and he has to work with Doctor Strange to fix it before it's too late.
Well, a plot might involve a device in the Pokemon world that can temporarily give trainers the ability to transform like Ben. But it has a side - effect of attracting some of Ben's most dangerous foes. So, the trainers and Ben must work together to fight off these threats while also dealing with the chaos that the newfound powers are causing in the Pokemon world. They could also learn from each other's fighting styles and techniques during the battles.
Maybe the Omnitrix gets malfunctioning and starts to attract Grimm. The RWBY team has to protect Ben while he tries to fix it. During this time, they could learn about each other's powers and cultures. Ruby might be fascinated by the different alien forms and how they can be used in combat, and Ben could be impressed by the Dust - based technology of RWBY's world.
One idea could be Gohan getting transported to the Marvel universe and teaming up with the Avengers. He could use his Saiyan powers to help them fight against Thanos. Maybe his energy blasts would be a new weapon against the powerful foes in the Marvel world.
One idea could be Sans from Undertale ending up in the Marvel universe. He could use his shortcuts and bone attacks to fight alongside the Avengers against a common enemy like Thanos. His laid - back attitude would contrast with the more serious heroes in Marvel.
One idea could be that Superman gets transported to the Marvel universe during a freak interdimensional event. He teams up with the Avengers to fight a common enemy like Thanos, who has somehow obtained Kryptonite. Their combined powers and different strategies would make for an epic battle and an interesting exploration of how the two universes' heroes interact.
Maybe in the fanfiction, Cortana could be integrated into Iron Man's suit. Her advanced AI could enhance the suit's capabilities even further. And they could work together to take on a new villain that is causing chaos in both the Halo and Marvel worlds. It would be a great mix of science - fiction elements from both universes.
One plot idea could be that the vampires from Twilight possess some of the superpowers from the Marvel universe. For example, Edward could have Wolverine's healing factor in addition to his vampire abilities. This would make him even more indestructible and add a new layer to his character.
One plot idea could be Naruto being transported to the Marvel Universe during a strange space - time rift. He might team up with Spider - Man. Naruto's ninja skills like shadow clones and rasengan could be a great asset in battles against Marvel villains. Another idea is that the Infinity Stones somehow end up in the Naruto world, and the Avengers have to go there to retrieve them, leading to a clash and then an alliance between the two groups.
One idea could be that the Infinity Stones are somehow connected to the Warp in the Warhammer universe. Maybe Thanos discovers a way to tap into the power of the Warp through the Stones, which brings the heroes of Marvel into conflict with the daemons and factions of Warhammer.
A fun plot could involve the Marvel heroes visiting the Soul Society. Captain America might be intrigued by the strict hierarchical structure and the sense of honor among the Soul Reapers. Meanwhile, Spider - Man could get into all sorts of trouble exploring the different districts of the Soul Society, and his quick - wittedness would be put to the test when dealing with the various spirits and Soul Reapers.