Seriously, nobody mentioning canon divergence where Levi's squad survives? That's fertile ground. I read one where instead of joining the Scouts, Y/N washes up injured on the streets and Levi's cleaning compulsion makes him fix her up, leading to this gruff, slow-burn caretaker dynamic that felt more authentic than most action fics. The tension came from him trying to maintain distance while she inadvertently becomes part of his messed-up domestic routine underground. It sidestepped the whole 'strongest soldier' power fantasy angle, which gets tired.
Another one I'm a sucker for is the 'enemies through circumstance' plot. Like, Y/N is a titan-shifter from a rival faction, captured and held by the Scouts, with Levi assigned as guard. The forced proximity and ideological clashes wrote themselves, moving from mistrust to a twisted respect, then maybe more. It works because it mirrors his history with Kenny—that familiar dance of violence and reluctant connection.