I've read a ton of these fics over the years, and honestly, the themes are pretty consistent but also kind of fascinating in how they're executed. The most obvious one is Luffy's absolute immunity to Hancock's usual charm—she's used to everyone falling at her feet, but he just doesn't see it. That creates this huge emotional gap for her to bridge, which writers love to explore. It's less about romance in a traditional sense and more about Hancock learning to be vulnerable and seen for who she is, not her reputation or her beauty.
A lot of stories delve into her past trauma from being a slave and how Luffy's simple, unwavering kindness becomes a healing force. He doesn't treat her like a broken thing to be fixed; he just accepts her. That often leads to protective themes on her end too—she's so powerful, but she's terrified of anything harming him, which creates a nice tension between her strength and her fear.
You also see a fair amount of 'found family' blending, with Hancock trying to fit into the Straw Hat dynamic, which is always awkward and hilarious. The emotional core usually isn't dramatic passion, but a slow, steady warmth. He's her sun, and she's learning what it's like to orbit something that gives light without asking for anything back. It's a specific kind of comfort fic, I guess.