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.
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 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.
A Nightwing crossover with the Flash would be exciting. Picture Nightwing trying to keep up with the Flash's speed during a battle against a speed - stealing villain. Or a crossover with Green Arrow, where they both take on a group of criminals in Star City and Blüdhaven, and have to combine their different fighting styles.
At the start, the NCIS agents would likely view Nightwing as an unknown vigilante. But as they get to know him, they'd see his value. For example, Tony from NCIS might crack some jokes at Nightwing's expense at first, but then be impressed by his detective skills. Abby would be fascinated by Nightwing's gadgets. And as for Nightwing, he'd adapt to the NCIS agents' more official and structured way of doing things. He might also share some of his knowledge about the criminal underworld in Gotham with them, which could be very useful in their investigation.
Red Hood is another good option. The two have a complex relationship, and a crossover could explore their past grudges and how they might overcome them to fight a common enemy. It could also show how their different approaches to justice collide and then reconcile.
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.
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.
A really cool storyline would be if there was a magical artifact in Gotham that was connected to the wizarding world, and Nightwing and Harry team up to retrieve it. Nightwing's acrobatic skills and knowledge of the city streets combined with Harry's magic would make for an exciting adventure. They could face off against both magical and non - magical foes along the way. Maybe the artifact is being sought by Voldemort and one of Gotham's super - villains, so they have to work together to stop it from falling into the wrong hands.
Nightwing and Captain America would have a great interaction. Cap could admire Nightwing's sense of justice and determination, which is similar to his own. They could bond over their leadership skills and discuss different strategies for protecting their respective cities and the world. And Nightwing could be in awe of Cap's iconic status and the values he represents. Also, it would be interesting to see Nightwing interact with Thor. The contrast between Nightwing's more grounded, acrobatic fighting style and Thor's god - like powers would be really cool. Nightwing might be curious about Asgardian culture while Thor could be intrigued by Nightwing's human tenacity.