Well, in such a crossover, the Avengers could meet the Big Hero 6 team in San Fransokyo. Maybe Iron Man would be impressed by Hiro's tech skills and offer to collaborate on some new inventions.
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.
A good plot could involve a villain from the Homestuck universe creating chaos in the Avengers' world. The Homestuck characters come to help, but there are misunderstandings at first between them and the Avengers. However, as they start to understand each other's powers and motives, they form a strong alliance to defeat the villain.
Captain America and Hiro would be a great character pair. Captain America represents old - fashioned values like honor and duty. Hiro is a young, brilliant inventor. Cap could inspire Hiro to use his inventions for more noble causes. And Hiro could introduce Cap to some of the latest technological advancements. They could also bond over their losses. Cap lost Bucky for a while and Hiro lost Tadashi. So they could understand each other's pain and work together to fight the bad guys in the crossover fanfic.
It could be really exciting. Maybe the Avengers could visit San Fransokyo and be amazed by the high - tech there. Hiro could show them Baymax and they might collaborate to fight a new super - villain that threatens both their worlds.
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 possibility could be that Ben Grimm, as The Thing, brings his unique strength and loyalty to the Avengers. His rocky exterior could be a great asset in battles, like providing cover for his teammates. He might also have some interesting interactions with the other Avengers, especially those with a sense of humor like Tony Stark. They could make some witty remarks about his appearance while also respecting his power.
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 key element is the character interaction. How Venom, with his anti - hero nature, interacts with the heroic Avengers. For example, his relationship with Spider - Man within the Avengers context can be very interesting.
One unique element is the combination of magic and superpowers. In normal stories, these are two very different concepts. But in a crossover fanfic, seeing Iron Man's tech interact with spells from Hogwarts is really cool.