A great plot for Halo 4 OC fanfiction could be an OC who is a scientist. They are researching the Halo rings and accidentally activate a dormant function that starts to cause strange things to happen. The OC then has to team up with the Master Chief and his team to stop the Halo from going haywire. They might also discover new things about the Forerunners along the way.
Another plot could revolve around an OC who is a Spartan trainee that doesn't quite fit the mold. They have a different way of thinking and approaching combat. In Halo 4, they end up being the key to solving a mystery related to the Prometheans. Maybe they find a way to communicate with the Prometheans and convince some of them to stop fighting the humans.
One idea could be exploring the origin story of a Forerunner OC. Maybe they were part of a special caste that had unique abilities. Another could be a story about an OC Forerunner's relationship with other well - known Forerunner characters, like the Didact. And a third idea would be to have an OC Forerunner deal with the fall of their civilization, showing their struggle and the choices they make during that chaotic time.
One idea could be that the Servants from Fate are transported to the Halo universe. For example, Saber could find herself in a battle against the Covenant. Her noble phantasm could be a powerful weapon against the alien foes. Another plot could involve Master - Spartan team - pairings. A Spartan could be paired with a mage to fight a common enemy that threatens both the Halo and Fate worlds.
One idea could be a Spartan from Halo ending up on Pandora. Maybe they team up with the Na'vi to fight off a common enemy, like a new type of alien that's more powerful and dangerous than the Covenant or the RDA. The Spartan could introduce advanced human technology to the Na'vi, which would change their way of life in some aspects. For example, they could use some of the shielding technology to protect their homes. Another plot could involve the transfer of consciousness, like in Avatar, but with a Halo - related twist. Perhaps a character's mind is transferred into an enhanced alien body, similar to the Elites in Halo.
One idea could be that the Master Chief is somehow transported to Skyrim. He might use his advanced combat skills and technology to deal with the dragons. For example, his energy shields could protect him from dragon fire. Another plot could involve the races in Skyrim getting access to Halo's weaponry and having to figure out how to use it. Maybe the Dwemer could reverse - engineer Covenant weapons.
Maybe in the fanfiction, Cortana could be integrated into Iron Man's suit. Her advanced AI could enhance the suit's capabilities even further. And they could work together to take on a new villain that is causing chaos in both the Halo and Marvel worlds. It would be a great mix of science - fiction elements from both universes.
One idea could be that Naruto characters are teleported into the Halo universe. They might use their ninja skills to fight the Covenant. For example, Naruto could use his shadow clone jutsu to confuse the enemy, while Sasuke could use his Sharingan to predict their moves. Another plot could involve Master Chief getting some of Naruto's chakra abilities, making him even more powerful in combat against the Flood.
Another plot could involve Noble 6 having their consciousness transferred into a Forerunner construct in a crossover. This would force them to navigate Forerunner technology and politics while still trying to uphold their UNSC values. They could form an unlikely alliance with a Forerunner AI to stop a rogue element within the Forerunner society.
Another plot could involve a character from RWBY being transported to the Halo Wars universe and having to adapt to the new environment. They could learn about the military strategies and advanced technology there and then bring some of that knowledge back to their own world when they find a way to return. For example, Ruby could use her speed and Crescent Rose in new and interesting ways while also learning to pilot a UNSC vehicle.
One interesting Halo OC fanfiction could be about a new Spartan who has unique abilities and how they interact with the existing Halo universe characters. For example, a Spartan with the power to manipulate energy in ways that the Covenant or UNSC have never seen before.
One interesting story could be about an original character (OC) who has unique armor abilities. Maybe they can blend into the environment in ways that regular Spartans can't. This OC might use these abilities to infiltrate Covenant strongholds undetected and gather crucial intelligence for the UNSC.