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.
Maybe in this crossover, the Avengers are investigating a series of strange crimes in New York that seem to have a connection to Gotham. They reach out to Nightwing for help. Nightwing brings his knowledge of Gotham's underworld and together they uncover a plot by a new super - villain who wants to take over both cities. This would lead to some really interesting team - up dynamics as the Avengers and Nightwing have different ways of operating.
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.
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.
One idea could be that Godzilla is drawn to the energy source of one of the Avengers' tech, like Stark's Arc Reactor. This causes chaos as Godzilla approaches the city where the Avengers are based. The Avengers initially see Godzilla as a threat and try to fight it. But then they realize that an even bigger alien force is using Godzilla's presence as a distraction. So, they have to find a way to communicate with Godzilla and work together to defeat the common enemy.
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 idea could be that Spider - Gwen somehow gets transported to the Avengers' universe. Maybe Doctor Strange's magic goes awry during an experiment. She could team up with Black Widow, sharing her acrobatic skills and they take on a group of villains together. It would be a cool dynamic as Gwen is more of a street - level hero while the Avengers often deal with bigger threats.
Imagine if Elena Gilbert was somehow connected to an Infinity Stone. This would draw the attention of Thanos, but also the Avengers. The Vampire Diaries characters would have to protect Elena while also dealing with the fact that the Avengers are now in their small town. There could be a lot of mistrust at first, but as they learn more about each other's motives and capabilities, they form an alliance to keep the Infinity Stone out of Thanos' hands.
Maybe Zelda herself could be transported to the Avengers universe. With her wisdom and magic, she could assist the Avengers in different ways. She could use her magic to heal the wounded Avengers or help Tony Stark understand some ancient technology that has similarities to her own world's tech. She could also provide strategic advice based on her experience dealing with Ganon in her own realm.
One idea could be that the Avengers are somehow transported to the wizarding world. They might be initially confused by the magic but then start to work with the Hogwarts students and teachers to fight a common enemy, like a dark wizard who has also harnessed some sort of Muggle - like technology to become even more powerful. For example, Voldemort could team up with Hydra remnants to create a super - weapon that threatens both the magical and non - magical worlds.
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.
One idea could be that during a mission crossover, Spider - Man gets injured while trying to protect a civilian from an unexpected attack by a powerful enemy. Maybe he takes a hit meant for an Avenger and his suit's technology malfunctions, leaving him vulnerable.