Eric might also see Tris as an asset to his own plans or goals. If he wants to gain more power or influence within their community, he might think that by saving Tris from Peter, he can earn her loyalty and have her on his side. And Tris is known for her strong will and skills, which can be useful to Eric.
Eric might also use his position of authority. If he's in a position of power within the story's setting, he could simply order Peter to stop. For example, if Peter is violating some rules of the group or faction they belong to, Eric could threaten him with punishment if he doesn't let Tris go.
Love for Lily. Since Harry is Lily's son, Snape might feel an obligation to protect him. He couldn't save Lily, but he can try to keep her son safe.
One motivation could be self - preservation. Draco might realize that if Voldemort kills Harry, it will lead to an all - out war that could destroy everything he knows. By saving Harry, he hopes to find a more peaceful solution and protect his own family and the wizarding world in a broader sense.
One common theme is the exploration of their relationship dynamics. Since they have a complex relationship in the original work, fanfictions often dig deeper into how they interact, whether it's as friends, rivals or something more.
One common theme could be power struggle. Given their characters, there might be stories exploring who has more power between them.
I'm not sure as I haven't read this specific fanfiction. It could potentially involve Tris being in some sort of danger from Peter, and 'four' (presumably a character) coming to her rescue. Maybe Peter has some malicious intentions towards Tris, like trying to harm her or get her into trouble, and Four steps in to prevent that.
Well, Naruto has a strong sense of justice. He can't stand to see anyone being bullied, especially someone he cares about like Sakura. He might also want to prove himself. By standing up to the bullies, he shows that he's not the weakling some people think he is.
Another theme could be the idea of forbidden love. If there are certain circumstances in the original story that make their relationship difficult or against the rules, this can be a popular theme in the fanfiction. Writers can then explore how they deal with this forbidden aspect, like sneaking around or fighting against the system for their love.
A theme that can't be ignored is the transformation. Sometimes Tris undergoes a transformation, either physically or in terms of her understanding of the world. It could be that she becomes more like a vampire or gains a new perspective on life. And the overarching theme of good vs. evil, with Eric and Tris often on the side of good, fighting against the more malevolent vampires or entities in the fanfiction.
One common theme could be the challenges they face as a couple during the pregnancy. It might include things like dealing with their different personalities while preparing for a new life together.