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.
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.
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.
There's also 'When Descendants Join the Avengers'. This fanfiction focuses on how the Descendants integrate into the Avengers' daily operations. It shows their growth as they learn from the more experienced Avengers. Characters like Carlos become more confident as he takes on tasks with Iron Man, and it's really heart - warming to see their relationships develop.
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.
Well, one possibility could be Deadpool driving the Avengers crazy with his non - stop chatter and crazy antics. He might constantly break the fourth wall in the middle of a mission, making the Avengers wonder what on earth he's talking about. For example, he could start making jokes about how Captain America's suit is so old - fashioned compared to his own while they're in the middle of a fight against a powerful villain.
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.
A common theme is teamwork. Since it's a crossover, different characters from different universes or groups have to work together. For example, in an Avengers and X - Men crossover, they need to combine their powers and strategies.
One great 'pjo avengers crossover fanfiction' is 'The Demigod Avengers'. It has an exciting plot where the demigods from Percy Jackson and the Olympians join forces with the Avengers. The character interactions are really well - written, like when Percy uses his water powers in a fight alongside Thor's lightning.