One idea could be that the Infinity Stones are scattered across different planets again. The Avengers and the Guardians have to team up to find them before a new villain does. Maybe Thanos has a long - lost relative who is out for revenge and wants to use the Stones' power for destruction.
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.
A great plot could be that a member of the Avengers gets lost in space and is found by the Guardians. The Guardians then have to bring this Avenger back to Earth, but on the way, they encounter a series of challenges that require the combined skills of both groups. Along the way, they form strong bonds and exchange knowledge about their respective worlds.
One idea could be a prequel story about how each member of the Guardians joined the team. For example, showing Star - Lord's early days as a scavenger and how he first came across the Orb. Another could be a story where the Guardians have to deal with a powerful new enemy that has the ability to control time, forcing them to race against the clock in a whole new way. Maybe a fanfic about the Guardians getting stranded on a strange planet and having to work with the local inhabitants, who have their own unique culture and powers, to repair their ship and escape.
An OC who is a shape - shifter could be interesting. They could infiltrate the enemy's ranks and gather intelligence for the Guardians. However, they struggle with their identity as they constantly change their form, and the Guardians have to help them find their true self within all the disguises.
One exciting plot twist could be that the Infinity Stones have a different origin in this fanfic. Maybe they were created by an ancient race that both the Avengers and Guardians have to discover together. This would force them to work closely and share their knowledge.
One idea could be a fanfic where the Guardians have to team up with an old enemy to face a new, greater threat. Maybe a powerful cosmic entity that is threatening to destroy entire planets. Another idea is a fanfic focused on the backstory of a particular Guardian, like Rocket, exploring his life before he joined the team in more detail.
I think the meeting between Thor and Groot would be fascinating. Thor, being an Asgardian god, and Groot, a sentient tree - like creature. Thor might be initially confused by Groot's limited vocabulary ('I am Groot'), but as he gets to know him, he would see Groot's strength and loyalty. Groot could also be intrigued by Thor's lightning powers. And their combined strength in a fight against a common enemy would be a sight to behold.
Definitely Thor and Star - Lord. Thor is this god - like being from Asgard, and Star - Lord is a half - human, half - alien space adventurer. Their personalities are quite different. Thor is more noble and straightforward, while Star - Lord is a bit of a scoundrel. But they both have a sense of humor and a love for adventure. They could have some great banter while fighting side by side.
Well, you could have a plot where the Guardians accidentally stumble upon a parallel universe. In this universe, their counterparts are evil. They have to figure out how to return to their own universe while also preventing their evil doppelgangers from crossing over. Along the way, they might learn some secrets about the true nature of the multiverse and their own roles in it.
Another cool fanfiction idea could be a time - travel story. The Avengers accidentally send themselves to the time when the Guardians were just forming. This leads to a chaotic but interesting adventure where they have to work together without ruining the future of both groups.