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 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.
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.
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.
Your OC could be a mechanic from a different planet who idolizes Rocket. They join the Guardians after impressing them with their skills in fixing a crucial piece of the Milano. This could lead to a story about the OC learning the ropes of being a Guardian, getting into arguments with Rocket about the best ways to upgrade their weapons and ships, and also developing relationships with the other members of the team. It could also involve a plot where the OC has to face their own fears when they are in a dangerous situation on a mission.
Well, you could have an OC that is from a long - lost civilization in the Guardians of the Galaxy universe. This OC might hold the key to a powerful ancient weapon or technology. For example, they could have a staff that can open portals to different dimensions. Or an OC who is a bounty hunter that initially goes after the Guardians but then ends up joining them. Also, an OC that is a musical prodigy from a planet where music has the power to heal or harm, and they use their skills to help the Guardians in various situations.
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.
A great Guardians of the Galaxy 3 fanfiction concept could be a time - travel story. Maybe the Guardians accidentally get sent back in time and have to interact with their younger selves or even their predecessors in the Guardians' legacy. They would have to be careful not to change the future too much while also trying to find a way back to their own time. Another idea is a fanfiction that delves deeper into the relationship between Nebula and Gamora. How they grow closer or face new challenges together. And a third could be a story where the Guardians discover a new planet full of strange and powerful artifacts that they must protect from various factions who want to use them for evil.
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 pre - Infinity War story where the Guardians try to steal an Infinity Stone from a powerful and unexpected enemy. Maybe a new race that has been secretly collecting powerful artifacts.
One idea could be a crossover where the Guardians accidentally enter the Star Wars universe. They might team up with the Rebel Alliance to fight against the Empire. Their unique powers and personalities would add a new dynamic to the Star Wars battles.