The Avengers Infinity Wars is about Thanos' quest to collect all six Infinity Stones. He believes that by wiping out half of all life in the universe, he can bring balance. The Avengers and their allies try to stop him. There are many battles across different planets. In the end, Thanos manages to get all the stones and snaps his fingers, which causes many beloved characters to turn to dust.
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.
The postman could be an unexpected hero. Imagine he's been delivering mail all over the city and has unknowingly witnessed some strange activities related to Thanos' minions. One day, he stumbles upon a piece of alien technology that gives him the power to disrupt Thanos' communication network, which turns out to be a vital part of the Avengers' victory.
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.
The plot of The Avengers Infinity Wars is mainly about Thanos' plan. He thinks the universe is overpopulated. So he goes after the Infinity Stones. The Avengers and other heroes try to protect the stones. But Thanos is too strong. He gets all the stones and does what he intended, which is a big disaster for the universe.
Hawkeye. He's often overlooked but could be really interesting in a fanfic. He could have a solo mission during the events of Infinity Wars where he discovers a secret about one of the Infinity Stones that no one else knows. Maybe he has to use his archery skills in a completely new and creative way to protect this knowledge from falling into the wrong hands.
The characters can be incorporated by simply having them show up in the Star Wars galaxy. For example, Thor could land on Tatooine with his lightning powers, causing quite a stir among the locals. His Asgardian technology could be a mystery to the Star Wars characters.
Well, imagine Thanos in the Star Wars universe. He could be a Sith - like figure, trying to gather all the kyber crystals (akin to the Infinity Stones). The Avengers could team up with the Jedi to stop him. The Hulk could be like a Wookiee in terms of his brute strength, and Thor's hammer could be seen as a sort of lightsaber - like weapon in the Star Wars aesthetic. There are so many possibilities for these two universes to collide in fanfiction.