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 Hanji discovers a new side of Eren during a mission outside the walls. Maybe Eren has a unique way of dealing with the Titans that Hanji finds fascinating, and this leads to a deeper bond between them as they study his abilities together.
One idea could be exploring Eren's family life as a baby. Maybe showing how his parents interacted with him in a more detailed way, like his mother singing lullabies. Another could be a story where baby Eren has some sort of premonition about the future in his dreams, which is a very unique take on his character.
One interesting idea could be exploring Eren's potential even as a baby in the context of the Attack on Titan world. Maybe showing how his early experiences, even as a baby, might shape his future determination against the Titans.
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.
Mikasa is often paired with Dark Eren. Given their long - standing relationship in the original series, it's natural for fanfiction to explore how she reacts to his darker side. She might try to bring him back to the light or be conflicted about her feelings for him.
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.
One interesting storyline could be them teaming up against a new, extremely powerful type of Titan that neither has faced before. They have to learn to trust each other's instincts and fighting styles completely.
One popular fanfiction idea could be a story where Eren has to team up with the Female Titan against a new, more powerful enemy in the world of Attack on Titan. Maybe they start off as reluctant allies, but as they face challenges together, they begin to understand each other better and develop a unique bond.