A powerful cult leader among the humans could be a great antagonist. They might see the godlike OC as a threat to their power and influence. They could spread lies about the OC, turning the people against them, and even try to assassinate the OC to maintain their control over the walled cities.
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 possible scenario could be Naruto using his godlike powers to quickly assess and counter the Titans. His ability to sense danger and move at high speeds would make him a formidable force against the Titans. For example, he could use his shadow clones to distract the Titans while he goes for the kill with a Rasengan.
Well, 'oc' usually stands for 'original character'. So it might mean that an original character in an Attack on Titan fanfiction experiences a fall. Maybe it's a physical fall like falling from a wall or a building in the context of the Attack on Titan world. Or it could be a metaphorical fall like a fall from grace or a fall into some sort of trouble within the story of the fanfiction.
The Kingpin might be a good choice. He could see the Titans as a new weapon to use in his criminal empire. He could form an alliance with some of the more corrupt elements in the Attack on Titan world, like some of the military commanders who are only interested in power. He would use his wealth and influence to control the Titans and try to take over both universes. Spider - Man and the heroes from Attack on Titan would have to fight against his well - organized and well - funded forces.
Eren could be an interesting main character in a friendly - titan fanfiction. He has a complex relationship with the titans already. Imagine if he discovers a friendly titan that changes his view of all titans. He could try to understand it better and maybe even learn to control other titans through the knowledge he gains from this friendly one.
It could be an original character created specifically for the fanfiction. Maybe another gamer - like figure who has opposing goals to the Godlike Gamer OC. They might compete for resources or power within the Fate universe.
A power - hungry warlord could be a great main antagonist. He might see the Queens Titan as a threat to his plans of total domination and constantly try to undermine or destroy it. He could have a large army and advanced weaponry at his disposal.
Eren could be an interesting choice. Since he has such a complex character arc already, making him the tenth titan could add another layer to his story. He could struggle with the new power and how it changes his view of the world and his mission.
One great fanfiction could be 'The Unseen Shifter'. In this story, the male OC has a unique shifting ability that sets him apart from the normal titan shifters. The plot is full of twists as he tries to hide his true nature from both the Survey Corps and the Titans. It really delves deep into the lore of Attack on Titan and creates an interesting character arc for the OC.
The plot needs to be engaging. It should blend the elements of the original 'Attack on Titan' series with the new ideas brought by the reincarnation concept. Like, how the OC's presence changes the course of events. Also, the relationships with the existing characters should be believable. Whether it's friendship, rivalry or romance, it has to feel natural within the context of the story. For instance, if the OC becomes friends with Eren, it should be because of some shared goals or mutual respect.