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.
The plot could involve a conflict within the group of titan shifters because of the tenth titan. Some might see it as a threat, while others believe it's the solution to end the war. The tenth titan's power could be something that disrupts the balance between the different factions. For example, it could have the ability to nullify the powers of other titans, which would make it a highly sought - after and dangerous power.
One idea could be an OC who is a Titan Shifter with a unique ability like being able to control the elements while in Titan form. For example, they could create ice storms or fire blasts. Another could be an OC who has to struggle with the morality of being a Titan Shifter in a world where Titans are seen as pure evil. They might try to find a way to coexist with the other humans instead of just fighting Titans.
A great idea for an 'Attack on Titan' permanent titan fanfic would be to focus on the relationship between a permanent titan and a human. It could be a story of an unlikely friendship or a situation where the human is trying to study the titan to gain some sort of advantage in the fight against the other titans. The fanfic could also delve into the emotional and psychological aspects of the titan, if it has any remnants of its former human self, and how it copes with its existence.
One idea could be exploring an alternate timeline where the main characters have different Titan powers. For example, Eren could have the Colossal Titan's power from the start. Another could be a story centered around a new character who joins the Survey Corps and has a unique ability to communicate with Titans on a deeper level. Maybe they try to find a peaceful coexistence with the Titans instead of just fighting them all the time.
One idea could be exploring an alternate timeline where Eren never became a Titan. Maybe the Scouts find a different way to fight the Titans, relying more on advanced technology and strategic warfare. Another could be a fanfiction centered around a new character who has a unique ability to communicate with Titans in a non - aggressive way, changing the whole dynamic of the battle.
One idea could be an OC who has a unique ability to communicate with the Titans. This could lead to a whole new way of dealing with the Titan threat, like trying to negotiate or understand their motives. Another plot could revolve around an OC from a different nation outside the Walls who has their own knowledge and skills regarding Titans. They could bring in new strategies and technologies.
Perhaps the fanfiction could focus on the reactions of the other characters. For example, Eren might have complex feelings about Historia's pregnancy, as it could symbolize a new hope for humanity in a way, or he could be worried about her safety in such a vulnerable state.
One idea could be exploring Dark Eren's internal struggle in more depth. Maybe he has to make really tough choices between his friends and his new, darker goals. Another could be a scenario where Dark Eren tries to recruit others to his side, and how they react to his changed personality.
One idea could be that the OC is reincarnated as a titan shifter in a different era of the Attack on Titan world. They could have memories of their past life and use that knowledge to change the course of events. For example, they might try to find a way to end the titan curse earlier.
A possible plot could involve Optimus Prime being mistaken for a new type of Titan at first. The characters in Attack on Titan initially fear him but then realize he's an ally. He could then help in fortifying the walls with his super - strong materials. This would create a new dynamic in the story as the people of the Attack on Titan world have to adapt to having this powerful being around and figure out how to work with him.