One idea could be a crossover between 'Endgame' and a fantasy world like 'Lord of the Rings'. Imagine the Avengers having to fight Sauron's forces in Middle - earth. Another could be a crossover with 'Star Wars', where the Avengers team up with the Jedi to battle a new threat that spans across galaxies.
First, decide which two (or more) universes you want to cross over. Then, think about a central plot that can bring the characters from both universes together. For example, if it's 'Endgame' and 'Doctor Who', maybe the Time Vortex is malfunctioning and affecting the Avengers' timeline. After that, start with an engaging opening scene, like the Avengers witnessing a strange time - related phenomenon that leads them to the Doctor.
One idea could be a crossover with the Doctor Who universe. The Avengers could team up with the Doctor to fight a common enemy that threatens both their universes. Maybe Thanos somehow gets access to Time Lord technology and the combined forces have to figure out how to stop him from using it to rewrite all of reality.
A great plot could be that Thanos somehow discovers the power of the Greek gods and tries to steal it for himself. He starts by attacking Camp Half - Blood. The Avengers, who are aware of some strange energy signatures coming from that area, go to investigate. There they meet Percy and his friends. At first, there is mistrust, but they soon realize they need to work together. Percy and his friends help the Avengers understand the magic and powers related to the Greek gods, while the Avengers offer their high - tech weaponry and combat skills. They then embark on a journey to stop Thanos from achieving his goal, facing many challenges along the way, including some Greek monsters and Thanos' minions.
Gandalf from The Lord of the Rings. Imagine him showing up in the Marvel universe, using his magic to help the Avengers. His wisdom and powerful spells could be a great addition, especially in a battle against a powerful foe like Thanos. He could also offer different perspectives on heroism and sacrifice.
To make the crossover believable, focus on the similarities between the two universes. Both the anime world and the Avengers' universe have characters with extraordinary powers. You can start by establishing a connection like a rift in space - time that pulls the characters from one world to the other. Then, make sure the characters' personalities are consistent. For instance, if an anime character is known for being honorable, they should act that way in the Endgame setting as well.
It enhances the viewing experience by building more excitement. You get to read all these different takes on what the trailer might mean for the movie, and it makes you more eager to see how the real movie plays out.
Another fanfiction idea could be a prequel to her role in Avengers: Endgame. It could show what she was doing in the cosmos just before she received the call to return to Earth. Maybe she was facing a different threat that was related to Thanos in some way, and this backstory could add more depth to her character in the movie.
One possible fanfiction could involve Spider - Man and Captain Marvel teaming up during the Endgame battle. Maybe Spider - Man uses his agility to distract the enemy while Captain Marvel blasts them with her powerful energy. They could also have some fun banter, like Spider - Man making a joke about how Captain Marvel can fly and he has to swing around.