One idea could be to cross the Avengers with the characters from Harry Potter. Imagine Iron Man trying to figure out how to use a magic wand, or Thor comparing his hammer to a magic broomstick. Another idea is to crossover with the Star Wars universe. Captain America leading the Rebel Alliance against the Empire would be really cool.
One idea could be a new character joining the Avengers. For example, a young mutant with the power to control time. This character could accidentally disrupt the timeline while trying to prove themselves to the team, leading to all sorts of chaos and adventures as the Avengers try to fix it.
One idea could be a crossover where Naruto gains Saiyan powers and teams up with the Avengers. He could use his newfound Saiyan strength and speed, combined with his ninja skills, to take on powerful villains. For example, against Thanos, Naruto could use his Rasengan enhanced with Saiyan ki energy.
One idea could be a crossover where the Avengers end up in the wizarding world of Harry Potter. They might be confused by the magic at first but then start to learn and adapt. For example, Iron Man could try to figure out how to incorporate magic into his tech. Another idea is that Harry Potter and his friends are brought to the Avengers' universe and they help the Avengers fight a new kind of threat that has both magical and non - magical aspects. Maybe Voldemort somehow teams up with a supervillain from the Avengers world.
A plot could involve the Avengers traveling to different dimensions through the disks and having to team up with alternate versions of themselves to defeat a common enemy. This would bring in a lot of interesting character interactions and new power combinations.
One idea could be exploring what if a minor character from the movie got the Infinity Stones instead. For example, a random Asgardian warrior. They might use the power in a completely different way, perhaps trying to restore Asgard rather than the more universal aims Thanos had.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
The fanfic 'Avengers meets the Fantastic Four' is also good. It plays on the similarities and differences between the two groups. The scientific genius of Reed Richards in the Fantastic Four is contrasted with Tony Stark's in the Avengers. And there are some hilarious exchanges between the more humorous characters like the Thing and Hulk. They also team up to face a threat that neither group could handle alone.
One idea could be that the Avengers discover a new form of happiness that is not based on victory but on self - discovery. They each go on a journey to find what truly makes them happy outside of their superhero personas.
One great Avenger crossover fanfiction is 'Avengers and the Justice League'. It's really exciting as it imagines what would happen if the Avengers from the Marvel universe met the powerful heroes of the DC Justice League. The story is filled with epic battles and interesting character interactions, like Iron Man and Batman sizing each other up, and Thor and Superman having a friendly competition of strength.
A Rambo crossover fanfiction could involve him crossing over into the Star Wars universe. Rambo's guerilla warfare tactics could be interesting in a galaxy far, far away. Maybe he helps the Rebel Alliance in a ground - based battle against the Empire. Or a crossover with the Predator series. Rambo would be a worthy opponent for the Predator, and the fight between them in the jungle could be epic. Also, a Rambo and John Wick crossover would be intense, two highly skilled fighters on the same side or perhaps initially at odds.