Well, a great plot for Avengers Battleground fanfiction could be a cross - over. Imagine characters from other superhero universes end up on the Avengers Battleground. There could be conflicts between different ideologies of heroism. For example, if the X - Men showed up, they might have different ways of handling the situation compared to the Avengers. Or, perhaps a group of young, inexperienced heroes are accidentally teleported to the Battleground. The Avengers then have to protect them while also fighting the main enemy. Also, a plot where the Battleground itself becomes sentient and starts changing the rules of the fight to test the Avengers would be really cool.
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.
One idea could be the Avengers facing off against a new, unjust system within their own superhero community. Maybe some heroes are being wrongly persecuted due to false accusations of using their powers for personal gain. This would create a lot of internal conflict as they try to clear their names and fight against the system.
One idea could be exploring the Next Avengers facing a new, powerful villain from another dimension. This villain could have unique powers that the young heroes have never encountered before, forcing them to quickly adapt and learn new strategies.
One idea could be that Kratos is somehow transported to the Avengers' universe. He initially clashes with them due to his different methods and values. For example, Thor might see Kratos as a rival in strength. But eventually, they team up to fight a common enemy like Thanos who has somehow gained the power of the Greek gods. Kratos could teach the Avengers about his brutal combat style and his knowledge of ancient magic.
One idea could be Bella has a unique power that the Avengers need to defeat a new, powerful enemy. For example, she can manipulate time briefly, which is crucial in a battle against a villain who can freeze time for long periods. Another could be Bella being from an alternate universe and she has to team up with the Avengers to stop the merging of her universe with theirs which would cause chaos. Maybe she has knowledge of a magic that the Avengers have never encountered before.