Man, this pairing kind of breaks my brain in the best way. I've spent more time than I'd care to admit scrolling through fics with that tag, and the common thread is this profound, unshakable understanding that borders on the metaphysical. It's rarely about romance in a traditional sense; it's about gravity. Luffy is her sun, and she's the one who can actually chart a course by his light.
Authors dig into the aftermath of Arlong Park, of course, but the really good ones go further. They explore the weight of her dream—to map the world—set against the sheer, world-bending force of his ambition. How does the cartographer make sense of the man who redraws the map of the possible just by existing? The emotional bond is shown through her quiet moments of calculation, not just of navigation, but of him. She's constantly recalibrating her understanding of their crew, their safety, their next move, with Luffy as the central, chaotic variable.
A favorite trope of mine is when a story has Nami, in a moment of crisis, not needing to explain anything. She just meets his eyes, and he gets it. He might not understand the politics or the math, but he understands her. That silent exchange, built on a thousand shared dangers, is the core of it. The fanfiction that nails this doesn't need grand declarations; it's in the way she hands him an orange slice without being asked, or how he stands just slightly between her and a perceived threat, even if that threat is just a nosy noble at a party.