A really engaging plotline might be that the reader is a paranormal investigator. They come to the location to study the strange occurrences related to Springtrap. At first, Springtrap is hostile, but as the reader delves deeper into the mystery, they find a connection. Maybe there's a past event that links them, and they end up working together to uncover the secrets of the place.
One interesting plotline could be the reader being a new employee at the haunted attraction where Springtrap is. They initially fear Springtrap but gradually discover there's more to his story. Another could be Springtrap protecting the reader from other malevolent forces in the haunted place.
A possible plotline is that Springtrap sees Deliah as a way to break free from his current trapped state. He may manipulate her at first, but as time goes on, he starts to develop real feelings for her. Meanwhile, Deliah is constantly in a struggle between her fear of Springtrap and her growing empathy for him.
Another theme is the exploration of an unusual relationship. Springtrap is not a typical love interest, so the fanfictions often focus on how the reader and Springtrap interact and form a connection that defies normal expectations. It could be a relationship based on mutual understanding in a very strange and dark setting.
One plotline could be a love triangle. The reader might be initially drawn to Light's charm and his seemingly noble goals as Kira, but as they get to know L better, they start to see his genius and his dedication to justice. This creates a conflict within the reader as they have feelings for both. Meanwhile, L and Light are also competing for the reader's affection while still being locked in their battle over the Kira case.
One interesting plotline could be the reader getting trapped in the haunted game world that Ben Drowned inhabits. They then have to find a way to escape while also dealing with Ben's unpredictable and often menacing presence.
One interesting plotline could be where the reader has some sort of hidden power and Max is the only one who can help them control it. For example, in a story, the reader discovers they can time - travel but it's really dangerous. Max, with his knowledge of superpowers, steps in to guide the reader. Another plotline could be a love triangle situation where Max, the reader, and another character from the Thunderman universe are involved.
One popular springtrap x reader fanfiction is 'The Encounter with Springtrap'. It has an interesting plot where the reader character stumbles upon Springtrap in an abandoned part of the pizzeria. The story does a great job of building tension between the two characters.
One interesting plotline could be where the reader is a new servant in the Phantomhive household and Sebastian takes a special interest in her, perhaps seeing her as a unique challenge. He might start to teach her the ways of being a proper servant while also being secretly intrigued by her different perspective.
A possible plot is that in the fanfiction world, there's a glitch that causes Springtrap and Foxy to swap abilities for a short time. This leads to humorous situations as they try to adapt to their new powers. Also, a plot where Springtrap discovers Foxy has a hidden talent, like painting or singing, and this changes his perception of Foxy completely.
A really unique plot might be that Springtrap and Nightmare are both pawns in a larger game. A mysterious entity is controlling their actions and they start to suspect it. As they try to break free from this control, they learn more about each other's true natures and form an unlikely alliance against the real enemy.
There might be a fanfiction where Springtrap has a change of heart due to Bonnie's kindness. Bonnie, being the more benevolent animatronic, tries to reach out to Springtrap who is initially resistant. As the story progresses, a strange bond forms between them, and they go through various adventures together, like exploring the secrets of the abandoned pizzeria at night.