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.
Maybe there could be a plot where a powerful villain has the ability to split the Avengers Prime into their individual selves from different timelines. So there are multiple versions of each Avenger running around. They have to find a way to reunite and stop the villain from causing chaos across all timelines.
A plot could revolve around the Avengers Prime traveling through time to prevent a future disaster that they somehow caused. Along the way, they meet different versions of themselves and have to make tough decisions about changing the timeline.
A great plot could be that a powerful artifact from the Transformers universe is found on Earth and it has the power to either save or destroy the planet. The Avengers initially think it's a new weapon or power source, while the Autobots know its true origin. They have to convince the Avengers of its importance and together protect it from the Decepticons and any other villains who want to use it for their own evil purposes.
A plot could revolve around a group of trainers in the 'pokemon prime' universe who are on a quest to find a legendary Pokemon that has been lost for centuries. Along the way, they face various challenges and form strong bonds with their Pokemon. They have to battle other trainers who also want to find the legendary Pokemon for different reasons, like power or fame.
One idea could be a story where Optimus Prime has to team up with an unlikely ally from another planet. Maybe a small, scrappy alien that has some unique abilities that can help in the fight against the Decepticons. This would add an interesting dynamic as Optimus has to learn to work with a very different being.
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.