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.
You can try fanfiction websites like Archive of Our Own (AO3). There are a large number of Avengers x Star Wars fanfictions on it. The search function is very useful. Just type in 'Avengers x Star Wars' and you'll get a bunch of results. Also, FanFiction.net might have some as well.
In Star Wars Avengers fanfiction, the 'hidden - heritage' trope can be seen often. Maybe a character discovers they have a connection to the other universe in their family history. The 'redemption' trope is also popular. A villain from one universe might find redemption by helping the heroes of the other universe. And let's not forget the 'love - across - universes' trope, where characters from Star Wars and Avengers fall in love despite their different origins.
One interesting crossover could be a story where the Jedi from Star Wars meet the Avengers. Maybe the Jedi could teach the Avengers about using the Force, and in return, the Avengers could show the Jedi some of their high - tech gadgets. Another could be a battle scenario where the Sith Lords team up with some of the Avengers' villains like Loki, creating a chaotic and epic showdown.
One idea could be that Naruto is somehow transported to the Star Wars universe during the Clone Wars. He could use his ninja skills, like his shadow clone jutsu, in unique ways on the battlefields. For example, his clones could be used to create diversions while the Jedi try to outmaneuver the Separatist droids.
In such a fanfiction, there would be a lot of culture clashes. The Avengers are used to their Earth - based battles and technology. When they meet the Star Wars characters, they'd have to adapt. For example, the Hulk might not understand the concept of the Force at first and just smash things around. But then, he could learn to use it in his own way. The Star Wars characters would also be shocked by the Avengers' powers. Luke Skywalker might be inspired by Iron Man's suit and try to create something similar for himself.
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 possible plot is that Thanos discovers the power of the Force and decides to get his hands on it. He teams up with some of the Sith Lords in the Star Wars universe. The Avengers then join forces with the Jedi and the Rebels to stop this new, even more powerful threat. Spider - Man could use his agility in the zero - gravity environments of some Star Wars planets, and Hulk could be a force to be reckoned with against the Sith's dark side powers.
In some fanfictions, you might see Kanan Jarrus training the Avengers in lightsaber combat. Since the Avengers have different fighting styles, learning the elegant and precise art of lightsaber fighting would add a new dimension to their skills. Also, imagine Thrawn strategizing against the Avengers. His brilliant military mind would pose a great challenge to the heroes.
A great idea for an Echo x Star Wars Rebels crossover fanfiction would be that Echo has some sort of encrypted data in his cybernetic enhancements that could be crucial for the Rebels. The Rebels have to figure out a way to access it without harming Echo. This could lead to a lot of character - development moments as they get to know Echo and his past better, and Echo gets to adapt to the Rebels' way of doing things.