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.
Another plot could focus on the psychological impact on the Avengers if they had to live in a world where Thanos won for a longer period before trying to reverse it. How would they cope with the loss and the new order? It could show their struggle on a more personal level, like Hawkeye dealing with his family being dusted.
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.
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.
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.
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.
There might be a fanfiction where the Avengers, instead of fighting Thanos in the way we saw in the movie, they actually manage to outwit him before he can even start collecting the Infinity Stones. They form an alliance with some unexpected allies from other galaxies, and together they foil Thanos' plan. This shows a different side of the Avengers' strategic thinking.
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 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.
Mantis. She has such unique powers but wasn't explored fully in the movie. In fanfiction, she could be the key to understanding the Stones better. Maybe she has a deeper connection to the Soul Stone that no one knew about.
One plot idea could be a character from RWBY getting their hands on the Infinity Gauntlet and having to deal with the power corrupting them. For example, Ruby might struggle with the new - found power and how it changes her perception of right and wrong.
One idea could be to have the cartoon versions of the heroes and villains swap powers. For example, Iron Man could end up with Thor's lightning powers and Loki could have access to Captain America's super - soldier strength. This would lead to really unique battles and character development as they struggle to control new abilities.