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.
A really cool endgame crossover fanfiction idea would be to combine it with 'Harry Potter'. Picture the Avengers arriving at Hogwarts during a dark wizard invasion. They would have to work with the students and teachers to defeat the dark forces using both magic and their superpowers. It would be fascinating to see how Iron Man would interact with the magical technology at Hogwarts.
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.
In some 'Avengers Endgame Fix It Fanfiction', there might be a concept of time travel without the glitches that occurred in the movie. Maybe they could have a more stable time - travel device which allows them to not only reverse the snap but also fix other events that led to Thanos' victory. This could involve changing things like Loki's escape with the Tesseract in a more beneficial way for the Avengers.
A post - Avengers Endgame fanfiction could center around the psychological impact on the survivors. Like Natasha's death still haunts the team. How does Steve Rogers' decision to go back in time and live his life with Peggy affect the others? It could also be about the search for new Infinity Stones. What if there are more Stones out there? And another idea is to write about the Avengers mentoring a new team of young heroes, passing on their skills and knowledge in a post - Endgame setting.
In a rewrite fanfiction, perhaps Loki could have faked his death all along. He could then return during the events of Endgame, bringing with him an army of Asgardians he had secretly been gathering. His cunning and strategic mind could add a whole new layer to the battle plans against Thanos and his forces.
Another plot could involve a new threat emerging that is related to the events of Endgame. The team has to piece together clues from the past events to stop this new danger. They might discover secrets that were hidden during Endgame that could change everything.
Imagine a fanfiction where Captain Marvel uses her energy - based powers in a completely new way during the final battle in Avengers: Endgame. Instead of just punching and flying around, she could manipulate the energy fields in a way that disrupts Thanos's entire army. This would be a creative take on her character and her abilities.
Perhaps a fanfiction where the Infinity Stones in the Endgame have an unexpected effect on certain anime characters. These characters start to develop new abilities or forms that are a blend of their anime selves and the power of the Stones. This could lead to a whole new dynamic in the fight against Thanos, as the Avengers have to figure out how to work with these newly - empowered anime allies.
Another idea could be that all the snapped heroes come back with new powers. This would create a whole new set of challenges for the Avengers as they try to integrate these newly powered heroes into their fight against Thanos. It would also make for some really exciting battle scenes.
One idea could be an OC who has the power to time - travel in a unique way, not like the Avengers' method. They could accidentally disrupt the events of 'Endgame' further while trying to help. For example, they might create alternate timelines where different villains gain power. Another plot could involve an OC who is a long - lost relative of one of the Avengers. They show up during the final battle and use their hidden powers to turn the tide. And a third idea is an OC who is from a different dimension entirely and has knowledge of how to defeat Thanos more easily, but they have to convince the Avengers to trust them first.
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.