One possible fanfic plot could be that Eren has been planning this for a long time. He might gather intelligence about the Colossal Titan's weaknesses. Then, in a fierce battle, he manages to get close enough to deliver a fatal blow. This could be a very emotional moment for him as well, considering the history between them.
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.
Historia in 'Attack on Titan' is significant as she represents the hidden secrets and power struggles within the walls. Her character development is a key part of the story's exploration of the society. Eren, being the main protagonist, is the one who initiates a lot of the action. His character embodies the themes of freedom and the will to fight against oppression. Together, they add different dimensions to the story. Historia's story arc is intertwined with Eren's, and their combined presence enriches the narrative.
In many fanfics where Eren dies in Attack on Titan, it can be a very emotional and thought - provoking experience. Some fanfics might focus on how his friends deal with his death. For example, Mikasa could be in deep mourning, and Armin might struggle with the loss while also trying to uphold Eren's ideals in a different way. It could also explore the political and social ramifications in the world of Attack on Titan after Eren's death, like how the different factions react to the end of the great threat he represented.
To be honest, I'm not 100% sure. Maybe it was one of the minor characters whose actions weren't as prominently featured. You might have to rewatch the movie to get a clear answer.