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.
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.
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.
Well, I know that some fanfiction platforms have a tagging system. Check those tags for 'tris and eric' and explore the ones that seem interesting. Sometimes, you might find hidden gems that way.
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.
One common theme is the exploration of forbidden love. Since Eric has a rather antagonistic role in the original story, the idea of him and Tris getting together goes against the norm. Another theme is redemption. Eric might be seen as trying to redeem himself in Tris's eyes. And there's also the theme of power dynamics as they both come from different positions of power within the factions.
One common theme could be conflict. Since Eric is hurting Tris, there would be the obvious physical or emotional conflict between them. This can create a lot of tension in the story.
There's also 'Eric and Tris: An Alternate Path' which presents an alternate universe for them. In this fanfiction, they face different challenges together and their bond forms in a more complex and interesting way compared to the canon story.