One possible plot could be that Batman's Gotham is threatened by a powerful villain who has some connection to the Avengers' universe. Maybe Loki decides to cause chaos in Gotham as part of a larger plan against Earth. Batman, being the detective he is, starts to investigate and soon realizes that this is no ordinary criminal. He then somehow gets in touch with the Avengers, perhaps through SHIELD's intelligence network. Together, they combine their skills and resources to take on Loki and his minions. Batman's stealth and strategic mind would complement the Avengers' superpowers.
The plot could involve a new super - weapon that is being developed in Gotham. This weapon has the potential to disrupt the entire planet. The Avengers, who are always on the lookout for threats to global security, hear about it. They send Iron Man to Gotham to investigate. Meanwhile, Batman has also noticed some strange activities related to this weapon. Iron Man and Batman initially clash due to their different methods, but they soon realize they need to work together. They then bring in the rest of the Avengers to stop the weapon from being activated and falling into the wrong hands.
The plot could start with Batman getting an invitation from SHIELD. They've noticed some strange activities in Gotham that seem related to a threat that could affect the whole world. Batman joins the Avengers. He quickly realizes that their methods are very different from his. But he uses his stealth and detective skills to gather information that the Avengers couldn't. For example, he discovers a hidden base of the villains by following a small clue.
Maybe in the fanfic, Batman's rogues gallery somehow teams up with some of the Avengers' villains. This forces Batman and the Avengers to work together. Batman could introduce his unique gadgets and combat style to the Avengers, while they could show him their superpowers up close. For example, Iron Man could be intrigued by Batman's utility belt and try to create his own version of some of the gadgets.
Another idea is that a villain creates a rift between the two universes. The Avengers and Merlin have to team up to close this rift. During their journey, they exchange skills and knowledge. Black Widow could teach Merlin some combat skills, while Merlin could show the Avengers how to use magic in a more strategic way. This way, they not only close the rift but also become stronger as a combined force.
One possible plot could be that the Avengers are somehow transported to the Arrow's universe. Maybe a villain in the Arrowverse has acquired a powerful artifact that creates portals, accidentally pulling in the Avengers. Then, they team up with Arrow and his team to fight against a common enemy, like a new super - powered threat that neither group could handle alone.
One possible plot could be that Daredevil and Batman are both on the hunt for a new criminal mastermind in Gotham. This criminal has a unique way of operating that blurs the line between legal and illegal in a way that challenges both of their moral compasses. They initially clash due to their different methods, but then realize they need to work together to take down this new threat.
Well, one possible plot could be that the gods in the Percy Jackson universe notice some strange energy disruptions caused by the Infinity Stones in the Avengers' world. Percy and his friends are sent to investigate. They meet the Avengers and at first there are misunderstandings, like the Avengers thinking Percy's powers are from some new form of alien technology. But then they realize they need to work together to stop a common enemy, perhaps a new powerful being that is drawn to the combined power of the demigods and the Infinity Stones.
One possible plot could be that a powerful villain from another dimension threatens both the DC and Marvel universes. Superman and the Avengers have to put aside their differences and combine their powers. For example, Superman's super strength and flight could be paired with Thor's godly abilities and Captain America's leadership. They could start off not trusting each other, but as the battle progresses, they learn to work together and defeat the villain in an epic showdown.
One possible plot could be that the Avengers are somehow transported to the Naruto universe. They team up with Naruto and his friends to fight a common enemy like Kaguya who has somehow obtained the Infinity Stones. The Avengers' powers and technology would be a huge shock to the Naruto world, and they'd have to learn to combine their skills to succeed.
A cool plot could be that a villain creates a device that can manipulate time. This device causes chaos as it starts to merge different timelines. Spider - Man gets caught in the middle of this temporal mess while on a normal patrol in New York. He then reaches out to the Avengers for help. The Avengers, who have experience with time - related issues from previous adventures, team up with Spider - Man. Thor uses his lightning to disrupt the device's power source, while Spider - Man swings in to physically disable it with the help of Black Widow's combat skills. In the end, they manage to restore the timelines and save the day.
Maybe the plot could involve a new type of mutagenic weapon that is being developed in Gotham. Wolverine is sent to investigate as it could pose a threat to mutants. He meets Batman who is also on the case as it could be used by Gotham's criminals. They start off distrusting each other. Wolverine doesn't trust this masked vigilante, and Batman is wary of Wolverine's wild nature. However, as they dig deeper, they find that a common enemy from their pasts is behind it all and they must combine their skills to stop him.