One possible plot could be that the Titans from Attack on Titan are somehow connected to the alien threats in Destiny. The Guardians could be sent to the world of Attack on Titan to investigate the sudden appearance of these giant creatures. They might initially struggle to understand the unique nature of the Titans, which are different from the enemies they usually face. Along the way, they could team up with the Survey Corps, sharing their advanced technology and combat skills. This would lead to some epic battles where the combined forces of Guardians and the Survey Corps fight against the Titans, while also trying to figure out the mystery behind their existence.
A plot could involve a character from Attack on Titan being transported to the Destiny universe. For example, Eren. He could be mistaken for a new type of Guardian due to his ability to transform into a Titan. The Vanguard would be both intrigued and cautious of him. Eren, on the other hand, would be in awe of the technology and different species in the Destiny world. He might learn about the Light and how it can be used to fight enemies. Eventually, he could use his Titan powers in combination with the powers of the Light to help the Guardians against a major threat, like a powerful Vex invasion that has somehow spread to the world he came from as well.
Well, perhaps the titans could be like the animatronics in FNAF. The Survey Corps could be trapped in a titan - infested area that has some strange FNAF - like mechanics. For example, the titans are only active at night and there are 'rules' to avoid them similar to the rules in FNAF games.
Perhaps Eren and his friends from Attack on Titan enter the Unordinary universe and are mistaken for having abilities. They then have to navigate the complex social hierarchy of the Unordinary world while also trying to find a way back to their own world. And the Titans might start showing up in the Unordinary world, causing chaos.
One possible plot could be that the Avengers are somehow transported to the world of Attack on Titan. They team up with the Survey Corps to fight the Titans. Iron Man could use his high - tech suits to take on the Titans from a distance, while Captain America leads the ground troops along with Eren and his friends. Thor could use his lightning powers to stun the Titans, making it easier for the others to attack.
One possible plot could be that Naruto and his friends are transported to the world of Attack on Titan. Naruto's ability to use chakra and his ninja skills could be used to fight the Titans in a unique way. For example, he could use his Shadow Clone Jutsu to create multiple clones to distract the Titans while the Survey Corps tries to attack their weak points. His Rasengan could also be a powerful weapon against the Titans' tough skin.
One possible plot could be that the Titans are actually some kind of ancient, corrupted Transformers. The Survey Corps discovers a hidden technology that has the power to control or at least influence the Titans. Eren, with his Titan - shifting ability, might have a special connection to this technology. The Autobots arrive on the scene, initially seen as a threat by the humans. But as they learn about the Titans, they team up with the Survey Corps to fight against this common enemy, using their advanced weaponry and Eren's unique power.
One interesting plot could be that the godlike OC has the power to control the Titans' minds. They could use this power to try and broker peace between the humans and the Titans. For example, they might start by stopping a Titan attack on a walled city and then communicate with both sides to find a common ground.
One interesting storyline could be where the fanfiction explores an alternate universe where the Titans have a different origin. Maybe they are not just mindless giants but are actually the result of a failed scientific experiment gone wrong in a more complex way than in the original series.
One possible interesting plot could be that the Survey Corps discovers a titan that has some sort of connection to the Kaiju from Pacific Rim. Maybe the titan has a similar biology or origin. They could then team up with the Jaeger pilots to study it and figure out how to defeat both the titans and the Kaiju. The Jaeger pilots could teach the Survey Corps about their advanced technology, while the Survey Corps shares their knowledge of fighting in a more primitive, yet effective way against the huge and dangerous enemies.
One possible plot could be that Naruto and his friends are transported to the world of Attack on Titan. Naruto's skills in ninjutsu would be very useful against the Titans. For example, his Shadow Clone Jutsu could be used to create distractions while the Survey Corps try to attack the Titans' weak points. Also, Naruto's ability to sense chakra could potentially help in detecting Titans from a distance.
Perhaps the friendly titan is a protector of a certain area outside the walls that the humans have not explored much. The story could be about the Survey Corps making an alliance with this titan. The titan guards the area from other dangerous titans, and in return, the humans help the titan when it gets injured. This could lead to a whole new understanding of the titan world and how they can co - exist with the friendly ones.