Zuko's redemption arc is basically custom-built for OC inserts. I think the most persistent theme is the outsider's perspective on his transformation from Fire Nation prince to Avatar ally. An OC often functions as a neutral party, someone who wasn't there from the beginning, and their journey from suspicion or hostility to trust mirrors the audience's own shifting view of him. It's a vehicle for re-examining his guilt and growth from a fresh angle.
Another huge one is the 'found family' dynamic, especially if the OC is from the Earth Kingdom or a non-bending background. You get these stories where the OC, maybe a refugee or a deserter, gets reluctantly folded into Team Avatar because of Zuko. The themes of healing from war trauma together, building a new identity outside of your nation's propaganda—that stuff gets heavy, but it's compelling because it ties back to the series' core ideas.
Less discussed but super common is the political marriage trope. An OC as a noble from a Fire Nation colony or a conquered territory, married to Zuko for alliance purposes. The tension between duty and personal desire, navigating the corrupt court, and trying to implement actual reform alongside him allows for a deep dive into post-war Fire Nation politics that the show only hinted at. It's less about wild adventure and more about the bureaucratic and social struggles of building peace, which can be surprisingly gripping.
Finally, there's the classic 'teacher/student' or 'bodyguard' dynamic. An OC as his firebending instructor, or a Kyoshi Warrior assigned to protect him, creates a built-in power imbalance and forced proximity. The themes here revolve around mentorship, vulnerability, and earning respect on a personal level rather than through royal title. It often explores Zuko's relationship with firebending itself—the anger, the control, the legacy of his family—through the lens of someone trying to teach him a healthier approach.