Another interesting plot could be a time - travel scenario within the 'breach of fate' world. A character goes back in time to try and prevent a certain 'breach' from happening, but in doing so, they create unforeseen consequences that make the situation even more complex.
The protagonist is always a popular choice. You can develop their character further, explore their unspoken thoughts and feelings, and put them in new situations. For instance, in a 'breach of fate' fanfic, the main character might face a moral dilemma that wasn't in the original story.
For a 'breach of trust' fanfic to be compelling, it needs a strong conflict that makes you care about the characters' relationships. Good world-building and a sense of moral ambiguity can also draw readers in. And of course, a satisfying resolution that ties up loose ends is important too.
Finding 'a breach of trust fanfic' can be a bit tricky. You could also join relevant fandom groups on social media platforms like Reddit or Discord. Members in these groups often share and discuss various fanfics, and they might be able to point you in the direction of some good 'a breach of trust' fanfics.
For Rex, you could develop his character by showing his more vulnerable side when he's around Breach. Maybe he's usually the tough leader, but with Breach, he allows himself to show his doubts and fears. For Breach, show how he matures through his interactions with Rex. For example, he could become more responsible.
I'm not sure specifically as 'breach novel' could refer to many different novels. It might be about a story where there is a breach of some kind, like a security breach, a breach in a relationship, or a breach of a social norm.
Maybe a fanfic where Rex discovers a hidden talent of Breach that no one else knew about. For example, Breach is an amazing artist but has been hiding it. Rex encourages Breach to show it to the world, and through this process, their relationship blossoms. It could also involve some challenges from the other members who don't believe in Breach's new - found confidence.