One of the biggest conflicts I see writers circle back to is the inherent tension between Nami's practicality and Luffy's raw, instinctive freedom. She's planning for tomorrow, he's living for right now—and that creates this beautiful friction where she's trying to build a stable future for the crew, maybe even a home, and he's just happy sailing wherever the wind takes them. You get stories where she's trying to get him to understand money or long-term consequences, and he fundamentally doesn't, but he trusts her completely anyway. That trust is the core, honestly; the conflict isn't about them not caring, it's about them caring in such wildly different languages.
Then there's the whole 'what if something threatens the crew' angle. A lot of plots use an external threat—a powerful enemy, a curse, a political scheme—that specifically targets their bond or forces one to choose between the other and the crew's safety. Luffy would obviously throw himself into the sea for any of them, but watching Nami wrestle with that, with her protective instincts over him clashing with her duty to the ship and the others, that's where the real emotional gut-punches happen. It's less about will-they-won't-they and more about how far they'll bend their own natures for each other.
I've also stumbled on a few that explore the conflict of legacy and dreams. Nami's dream is so tied to a map, to something tangible she can create and leave behind. Luffy's is... an abstract crown at the end of a journey. How does a relationship even fit into that? Stories that grapple with that, with the idea that maybe after finding the One Piece, Luffy just keeps sailing, while Nami might want to plant roots somewhere—that's a more melancholic, bittersweet conflict that really gets under my skin.