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.
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 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 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 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.
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.
You could also write a fan fiction where the Avengers use time travel in a different way. Instead of going back to collect the Stones, they go back to stop Thanos from ever getting the idea of the Snap. This would involve changing key events in Thanos' past, like preventing him from meeting Gamora or finding out about the Infinity Stones.
Maybe Harry could use some of his magic skills to assist the Avengers in a battle against a powerful enemy. For example, he could use a spell to freeze the bad guys momentarily while the Avengers strike.
In a 'naruto is an avenger fanfiction' plot, Naruto could be brought to the Avengers' world because of a strange rift in the multiverse. There, he meets Captain America and is intrigued by his sense of justice. Naruto joins the Avengers in their fight against Loki, who is trying to use Naruto's chakra for his own evil plans. Naruto has to adapt to this new world while also showing the Avengers the power of friendship and determination, which are key elements in his own story.
One possible plot could be that Stiles is a huge Avengers fan in his own world, and through some magical or scientific mishap, he gets to meet his heroes. But it's not all fun and games. He has to prove himself worthy of being part of their team. He could also bring in some of his own world's technology or ideas that help the Avengers face a new and powerful threat.