One interesting plot idea could be that in the alternate timeline, Tony Stark never becomes Iron Man. Instead, he devotes his life to clean energy research and Pepper Potts takes on a more superhero - like role, using Stark tech to fight crime. Another idea is that the Avengers lose the Battle of New York in this timeline, which leads to a dystopian world where the Chitauri rule, and the remaining heroes have to form an underground resistance.
An exciting plot idea for an Avengers alternate timeline fanfiction might be that the Infinity Stones are discovered and used in different ways. For example, instead of Thanos collecting them, a group of benevolent beings from another dimension use the stones to create a utopia on Earth. But this utopia has some unforeseen consequences. The Avengers then have to decide whether to support this new world order or try to return things to the way they were, which causes a rift within the team.
Well, a plot idea could be that in the alternate timeline, Thor lands on Earth as a child. He grows up among humans, learning their ways and values. When the time comes for the big threats, he has a different perspective on how to handle things. Maybe he tries to use diplomacy more than brute force at first. Also, in this timeline, Loki could be a hero instead of a villain. He could have been influenced by the kindness of the humans he met early on.
Another idea could be that all the snapped heroes come back with new powers. This would create a whole new set of challenges for the Avengers as they try to integrate these newly powered heroes into their fight against Thanos. It would also make for some really exciting battle scenes.
One interesting plot twist could be if Harry was sorted into Slytherin instead of Gryffindor. This would completely change his relationships and the way he is perceived in the wizarding world. Another could be if Voldemort had a change of heart early on and decided to abandon his dark ways. It would be a huge shift in the entire story's dynamic.
One interesting concept could be a timeline where Naruto fails to bring Sasuke back. This would lead to a very different Konoha, with Naruto perhaps being more jaded and Sasuke's actions having far - reaching consequences for the ninja world. Another concept might be a timeline where the Akatsuki actually succeeds in their goals, which would completely reshape the power dynamics and politics of the ninja world.
Some interesting plots in RWBY alternate fanfiction could involve an alternate reality where the Dust doesn't exist. This would completely change the way combat and technology work in Remnant. Hunters and Huntresses would have to rely more on their physical abilities and semblances. Or there could be a story where Team RWBY is formed much earlier in their lives, like as children, and they grow up together facing different challenges than in the original story. There could also be a fanfic where the world of Remnant is invaded by an outside force from another dimension, and the RWBY characters have to adapt quickly to this new threat.
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.
One idea could be that the mutants and the Avengers have to team up against a common enemy that is immune to their individual powers. So they must learn to combine their abilities in new ways. For example, Wolverine's healing factor could be used to keep an injured Avenger alive during a long battle while Iron Man figures out a way to defeat the enemy.
One idea could be that Harry Potter is magically transported to the Avengers' universe. He could use his magic to help the Avengers in battles, like creating protective shields with spells. Another idea is that the Avengers are somehow sent to Hogwarts, and they have to learn about magic while also dealing with threats in the wizarding world. For example, Iron Man might try to figure out how to integrate his technology with magic.
One idea could be exploring the family dynamics when the Avengers have to take care of a young superhero in training. It would be fun to see how they each contribute to the kid's growth, like Tony Stark teaching about technology and Captain America instilling values.
One idea could be the Avengers facing off against a new, powerful villain who has the ability to turn their own powers against them. This would lead to some really intense battles and strategic thinking on the Avengers' part.
One idea could be that the adopted character has a special power that the Avengers initially don't understand. This power could be something like the ability to communicate with animals, which helps them during a mission where they need to track a villain through a forest. Another could be that the character has a connection to an ancient artifact that the Avengers are also interested in for some reason, and this connection leads to a series of adventures.