A plot could involve a powerful sorcerer in the Marvel Universe accidentally opening a portal to Nightwing's world. The Avengers and Nightwing come together when they discover that an ancient evil from both their worlds is planning to use this portal to conquer both universes. Nightwing's knowledge of the streets and his combat skills would be valuable to the Avengers as they try to stop this impending doom. He could also form interesting relationships with the different Avengers members, like a friendly rivalry with Hawkeye over who has better marksmanship skills or learning magic from Doctor Strange.
Well, perhaps in this fanfiction, there's a multiverse rift that pulls Nightwing into the Avengers' world. The Avengers initially mistake him for an enemy, but after some misunderstandings are cleared up, they realize his skills. Together, they face a new threat created by a combination of Hydra and some of Nightwing's rogues gallery who have also crossed over. Nightwing could teach the Avengers some stealth tactics while they show him how to handle more powerful cosmic threats.
One idea could be that Nightwing is transported to the Avengers' universe through some sort of interdimensional rift. He then has to team up with the Avengers to fight a common enemy, like Thanos who has somehow acquired some of Batman's tech. Nightwing's acrobatic skills and detective abilities would be a great addition to the Avengers' power set.
Perhaps a plot where Nightwing is framed for a crime in the Avengers' world. The Avengers initially think he's the bad guy but as they start to investigate, they realize there's a much bigger conspiracy at play. Nightwing, with his experience in being wrongly accused in Gotham, teams up with the Avengers to clear his name and stop the real villains. This would involve a lot of action as Nightwing has to prove himself to the Avengers while also fighting off the bad guys who are trying to stop them from uncovering the truth.
One possible plot could be that Nightwing is somehow involved in a case that crosses over into the NCIS jurisdiction. Maybe there's a criminal organization dealing with both high - tech gadgets (Nightwing's area of expertise) and military - related crimes (NCIS). The NCIS team initially suspects Nightwing but then realizes he's an ally and they team up to take down the bad guys.
One possible plot could be that they team up to stop a powerful villain who has the ability to travel between their two cities. Maybe this villain is stealing tech from Gotham and super - powers from New York. They start off not trusting each other as they have different methods. Nightwing is more stealthy and trained in combat, while Spider - Man is more quippy and uses his webs in a very acrobatic way. But as they face challenges together, they learn to work as a team and combine their skills to take down the bad guy.
One possible plot could be that the Shikon Jewel gets scattered across the Marvel universe. Inuyasha and his friends, like Kagome, have to team up with the Avengers to retrieve the pieces. Iron Man might be intrigued by Inuyasha's sword and try to study it, while Captain America could admire Inuyasha's fighting spirit. They'd face villains who also want the power of the Shikon Jewel, creating an epic adventure.
One possible plot could be that the Voltron team gets transported to the Avengers' universe. They might initially be seen as threats, but then they realize they need to team up to fight a common enemy, like a powerful alien force that's too much for either group alone. The Voltron lions could combine in the middle of New York City, creating an amazing spectacle.
A really interesting plot could be a time - traveling crossover. Supergirl goes back in time to the era of the original Avengers. She helps them in a battle that changes the course of history. In return, the Avengers help her find a way back to her own universe. During this time, she forms a special bond with Thor, as they both have superhuman strength and a connection to otherworldly powers.
One possible plot could be that the Master Chief is somehow transported to the Avengers' universe. He teams up with Iron Man to stop a new threat that combines Covenant and Hydra technology. Their combined skills in combat and technology would make for an exciting adventure.
One possible plot could be that Superman is somehow transported to the Marvel universe where the Avengers operate. He initially clashes with them due to misunderstandings. For example, his super strength might be seen as a threat. But then they realize they can team up to fight a common enemy, like a powerful alien or a super - villain who has the ability to manipulate time. They combine Superman's powers such as heat vision and flight with the Avengers' diverse skills like Iron Man's technology and Captain America's leadership to save both universes.
It could involve different timelines of the Avengers coming together. Maybe the Avengers from the original team meet their counterparts from an alternate universe where things like Tony Stark never became Iron Man in a different way. They have to team up to fight a common enemy that threatens all the universes.