Honestly, I’ve been combing through Archive of Our Own for this pairing for years, and the tag is less crowded than you’d hope. A standout that still lives in my head is 'A Thousand Miles from Sunday'. It’s a post-Enies Lobby AU where Robin doesn’t rejoin the crew immediately, and Franky seeks her out under Water 7. The author nails their voices—Robin’s dry wit and Franky’s over-the-top earnestness somehow mesh perfectly.
What I love is how it treats their shared history of trauma not as a dark cloud but as a quiet understanding. Franky building her a study that transforms into a panic room isn’t played for laughs; it’s this profoundly caring gesture. The romance is a glacial slow burn, mostly in shared silences and Franky showing off his latest bizarre inventions to make her smile. It’s less about grand declarations and more about two very broken, very practical people finding a weird, comfortable rhythm.
I’d also throw in 'Super Structural Integrity'—a much shorter coffee shop AU that’s unexpectedly charming. The premise is silly, but the characterization is sharp. Franky as a struggling custom bike mechanic and Robin as a grad student who keeps visiting his shop for the terrible coffee just to watch him work is oddly believable. The ending feels a bit rushed, but the banter is on point.