One exciting plot could be that the Guardians and the Avengers have to team up to stop a powerful cosmic villain who is collecting powerful artifacts from both Earth and other planets in the galaxy. The Guardians' knowledge of the galaxy and the Avengers' understanding of Earth could be combined to track down the villain.
Maybe the plot could involve a threat that is causing rifts in space - time. The Guardians, with their experience in inter - galactic travel, and the Avengers, with their scientific minds like Tony Stark, come together. They initially have trouble getting along due to their different ways of doing things. But as they start to understand each other, they use their unique abilities, like Star - Lord's piloting skills and Captain America's leadership, to close the rifts and save the universe.
An interesting plot could be centered around a new energy source that has been discovered. This energy source is extremely powerful but also very dangerous. The Guardians come across it during their travels and the Avengers learn about it on Earth. Both groups realize that there are bad guys trying to weaponize it. So, they join forces. Drax's strength and Hulk's power are used for combat, while Gamora and Black Widow work on the stealthy side of operations. Rocket provides the tech - savvy support, and Thor uses his Asgardian knowledge to figure out how to safely contain or destroy the energy source.