Another theme could be redemption. If Peter has done some bad things in the original story, in the fanfiction he might be on a path of redemption with Tris being a big part of that journey. Tris could be the one who helps him see the error of his ways and he tries to become a better person for her.
Adventure is also a theme. They might be on some sort of journey together in the fanfiction, facing challenges side by side which in turn strengthens their bond. For example, they could be on a quest to save their world or a particular group of people. And this adventure also gives them the opportunity to explore their feelings for each other more deeply.
Trust is a big theme. Since they start off on rather rocky terms, fanfictions often focus on how they build or break trust. Also, the theme of self - discovery is common. As they interact more, both Peter and Tris learn more about themselves and their capabilities.
In peter x tris pregnant fanfiction, a common theme might be the protection of the unborn child. Given that they are in a fictional world which might have its own dangers, they would need to ensure the safety of the baby. There could also be themes around how their friends and the community react to the pregnancy, whether it's with support or some form of conflict.
Love is a very common theme. Their relationship is often explored in more depth, showing their devotion and the challenges they face as a couple.
One common theme could be power struggle. Given their characters, there might be stories exploring who has more power between them.
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 struggle. Given the name 'Four Hurts Tris', it might involve the challenges and hardships that Tris faces, perhaps related to her relationship with Four or other aspects of her life in the story's universe.
Another common theme is the impact of pregnancy on their roles in the society they live in. Maybe they have to deal with different expectations from their friends or the community. Some fanfictions might also explore how they balance their responsibilities as future parents with their existing duties in their post - Divergent world. It could be that Tris is still involved in some sort of leadership role, even while pregnant, and Four has to adjust to support her in different ways.
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.
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.