One idea could be that in the fanfiction, instead of Thanos getting all the Infinity Stones, the Avengers manage to steal some of them first. They could then use the power of the stones they have to create alternate versions of themselves to fight on different fronts against Thanos' forces.
A really interesting plot could be based on the idea of time travel within the context of 'The Avengers Infinity Wars' fanfiction. What if the Avengers found a way to go back in time and change certain events that led to Thanos getting the Infinity Stones? They could try to prevent the destruction of Xandar where the Power Stone was first retrieved, or stop Loki from stealing the Tesseract on Earth, which was a crucial step in Thanos' plan.
One plot idea could be that the Avengers are somehow transported to the Star Wars universe. They have to team up with the Jedi or the Rebels to fight a common enemy, like a new Sith Lord who has powers similar to some of the Avengers' villains. Another idea is that the Infinity Stones are discovered in the Star Wars galaxy and both the Avengers and Star Wars characters have to figure out what to do with them.
A plot could involve a group of rebels from different species uniting to fight against the Infinite Empire. They could discover an ancient weapon or power that gives them an edge in the fight. This would include adventures across different planets, meeting new allies, and facing dangerous enemies along the way.
Another plot could focus on the relationship between Thanos and his adopted daughters in a more in - depth way in the context of the Infinity Wars. Maybe there was a time when they were planning a rebellion against him before he even started his quest for the Stones. This could add a whole new layer of backstory and drama to the events of the movie.
Another concept is that a new threat emerges that requires both the skills of the Avengers and the powers of the Star Wars characters. This new enemy could be a powerful being that can manipulate time and space. The Avengers' superhuman abilities combined with the Jedi's connection to the Force would be needed to defeat it. Black Widow could use her espionage skills to gather intelligence within the Star Wars universe, while Spider - Man (if he's part of the Avengers in this fanfic) could use his agility to navigate the complex terrains of different Star Wars planets.
A really cool plot idea could be that the Avengers are watching Endgame in a sort of 'alternate reality cinema' where the events in the movie start to bleed into their real world. So they have to stop the movie from fully playing out to prevent chaos in their own universe. This would involve a lot of action and strategic thinking as they try to stop things like Thanos from emerging from the screen while also trying to understand the significance of what they're seeing in the movie.
Perhaps the Avengers mistake one of the Horsemen for a new super - villain. As they engage in battle, they soon realize that the Horsemen is actually after a powerful artifact that a real villain in the Avengers' world has obtained. So they form an uneasy alliance to retrieve the artifact.
Maybe in the fanfiction, the multiverse is collapsing and the Avengers of different universes are brought together by a mysterious force. They find out that an experiment gone wrong in one of the universes has caused this. So they have to travel through different universes, each with its own unique versions of the Avengers and villains. Along the way, they discover new powers and abilities that they can use to try and reverse the damage and save the multiverse.
One interesting plot could be the Avengers facing a new, super - powerful villain made of an unbreakable steel - like substance. They have to figure out how to defeat it without their usual methods working.
One interesting plot could be the Avengers facing an ancient, all - powerful cosmic entity that threatens to destroy all of existence. They have to combine their powers in ways they never thought possible, like Iron Man's technology with Thor's magic.