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.
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.
Well, I'm not sure specifically about 'peter x tris pregnant' fanfiction. However, in general, when looking for a particular type of fanfiction, it's a good idea to check fanfiction forums as well. Sometimes fans discuss and recommend specific stories there. Also, make sure to read the reviews if you find any stories, as they can give you an idea of whether it's worth reading or not.
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.
Well, there's 'Peter and Tris: A New Beginning'. In this fanfiction, it starts from a point where they have to team up for a special mission, and through that, their relationship gradually changes. There are also some that focus on their backstories and how it could have influenced their interactions in the main story.
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.
You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find some 'tris chooses peter' fanfiction there. Just use the search function and type in relevant keywords.
There might be a fanfiction where Tris chooses Peter in an alternate universe where the whole concept of the factions is a bit more fluid. In this story, they could be working together to change the society from the inside. Their relationship would be full of twists and turns as they try to navigate this new world while dealing with their feelings for each other.
In the fanfiction 'four saves tris from peter', the most prominent characters are Four, Tris, and Peter. Four is likely to be a brave and capable character, with qualities that enable him to rescue Tris. Tris is probably a character in need of help, perhaps she's in a weak or dangerous position. Peter, on the other hand, seems to be the source of trouble, the one who poses a threat to Tris, which prompts Four to take action.
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.