I always felt the core tension between Byakuya and Rukia in fanfiction isn't just about the obvious stuff like the execution order or him adopting her. Those are plot points, sure, but what writers keep coming back to is the foundational disconnect in how they perceive duty and family. Byakuya's entire world is built on rules, honor codes, and the unbearable weight of legacy—first his promise to Hisana, then his duty to the Kuchiki clan, then his role as a captain. Rukia, having grown up without that structure, interprets duty as something more personal and immediate, like protecting her friends or doing what she believes is right even if it breaks protocol. That creates a constant low-grade friction where they're both trying to do the 'right' thing but from galaxies apart.
A lot of plots I enjoy dig into the aftermath of that. Like, a story where Rukia makes a command decision that saves the day but technically violates some ancient Soul Society law, and Byakuya has to navigate being proud of her strength while also being the institution that would punish her for it. The emotional conflict isn't anger; it's this profound, quiet anguish where his affection for her wars with the very system that defines him. You see it in fics that explore his POV during the Hueco Mundo arc—the guilt of his initial failure layered over his rigid understanding of how to protect someone. He thinks protection means keeping her within the safe, gilded walls of the clan compound, and she thinks it means letting her fight beside him.
Some of the best ones I've read completely ignore external threats and just sit with the two of them trying to have a conversation. The conflict is in the silences, the things they can't say because their respective traumas and social positions make direct communication feel impossible. Is he stern because he doesn't care, or because he cares too much and has no other language? Does she defer to him out of respect or residual fear? Untangling that knot is where the real drama lives, far more than any staged rescue mission.