One interesting plot idea is Sonic being mistaken for a new superhero by the Avengers. He has to prove himself and his abilities to them. Another is when the Avengers' world is invaded by Sonic's enemies, and they have to work together to defeat them.
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.
A possible plot for 'sonic meets sonic boom fanfiction' is that a strange rift in the dimensions brings Sonic and his friends from the classic Sonic world into the Sonic Boom world right when they are about to have a big celebration. However, the Sonic Boom characters are initially suspicious of these new arrivals. But as they start to realize that a common enemy is behind the rift, they join forces. Sonic and Sonic Boom Sonic could have a friendly competition to see who can defeat the most badniks during the adventure, which also helps them bond and understand each other's strengths better.
One plot idea could be Dark Sonic having to face a new villain that is even more powerful and is threatening to destroy his world. He has to find a way to control his dark powers even more to defeat this foe.
Perhaps the Avengers mistake one of the Horsemen for a new super - villain. As they engage in battle, they soon realize that the Horsemen is actually after a powerful artifact that a real villain in the Avengers' world has obtained. So they form an uneasy alliance to retrieve the artifact.
Maybe in the fanfiction, the multiverse is collapsing and the Avengers of different universes are brought together by a mysterious force. They find out that an experiment gone wrong in one of the universes has caused this. So they have to travel through different universes, each with its own unique versions of the Avengers and villains. Along the way, they discover new powers and abilities that they can use to try and reverse the damage and save the multiverse.
One interesting plot could be the Avengers facing a new, super - powerful villain made of an unbreakable steel - like substance. They have to figure out how to defeat it without their usual methods working.
One interesting plot could be the Avengers facing an ancient, all - powerful cosmic entity that threatens to destroy all of existence. They have to combine their powers in ways they never thought possible, like Iron Man's technology with Thor's magic.
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 a new character, perhaps a powerful sorcerer, arrives in the Avengers' world. This character has a unique power that attracts the Avengers in different ways, leading to a harem - like situation. For example, Thor might be intrigued by their strength in magic which he respects, while Black Widow could be interested in their stealthy and strategic use of power. Another plot could involve a time - travel accident where an Avenger from the future comes back and ends up in a complex relationship with the current Avengers team, forming a harem.
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.