Ah, the endless debate between 'sworn rivals' and 'secretly pining idiots'—it writes itself, really. I've noticed a strong pattern leaning into 'mutual pining with a side of violent tension,' where neither will admit a thing until some life-or-death scenario forces their hand. The 'fake dating' trope gets a workout, often with the crew betting on them or needing a cover for a mission. What gets me is how often writers use cooking as a love language for Sanji; the way he'll make some absurdly specific meal for Zoro after a fight, and Zoro will grunt and eat it without comment, is practically canon at this point. Underneath all the bickering, there's a bedrock of absolute, unshakeable trust—that they'll always have each other's back in a fight—which is where a lot of the heavier, more angsty fics dig in. A lot of authors seem fascinated by what happens when that trust is fractured, or tested in a way their rivalry can't paper over.
Another huge one is exploring their contrasting codes of honor—Zoro's unwavering, stoic devotion to his captain and his dream versus Sanji's chivalry and his own complicated loyalties. Fics that pit those philosophies against each other, where Zoro has to understand why Sanji would sacrifice himself for a 'stupid' reason, or Sanji has to grapple with Zoro's sometimes brutal pragmatism, are always the most gut-wrenching to read. They often end with a quiet understanding, not a grand declaration, which feels true to them.