Here's a crackpot theory: Xebecs had the Mero Mero fruit, and he was handsome af.
I'm not really invested in it, because I've always thought he'd have something a bit more grotesque/evil, but here are the reasons for this alternative:
Oda loves his metaphors and small hints, and after seeing Blackbeard say that he's been after Hancock's fruit for a while, it made me think on the whys.
I recently remembered a couple of things that could make this fun to think about:
When Luffy got hit by the Mero Mero beams, he mentioned the resemblance with Foxy's beams. Foxy was the character who first mentioned Beehive and the Davy Back Game, and we later found out that Rocks were associated with both. Something that's been making me curious is the reason for Roger and Garp to defend the Celestial Dragons, when the Government seem to have powerful weapons at their disposal. It'd be interesting if he managed to seduce an important figure into giving him whatever he was looking for in God's Valley. It could also be that more than seducing, an awakened form of the fruit could actually manipulate people, or something along those lines. Roger and Garp had to intervene in order to prevent that.
Also, Hancock's sisters reminded me of Orochi/Kaido, perhaps as metaphors. This was back during the first half, but one of them turned her hair into 8 snakes, similar to the Yamata no Orochi, and the other one became a burning snake, kinda like Kaido in his final attack. We know one of those fruits was given to Kaido during the God Valley incident, but Orochi's fruit was given to him by the old hag, who seemed to know quite a lot about Rocks. (she transformed into Shiki, and told Kaido to come to Wano). We don't know who Xebec's closest retainers were, but perhaps the previous users of those fruits.
I kinda like this manipulative/seductive, perhaps even promiscuous narrative. He tried to charm his way into being King of the World, but he also had the strength to make it happen. It could also make way for a lot of relevant half-siblings. The Devil is usually seductive.
- Shakky has been theorized about quite a lot, and a popular one is that she was part of Rocks. We don't know her full name, but I could see Hancock being Rocks/Shakkys grandchild. She's quite different from her sisters, and he has Conquerors Haki, while the other ones don't. It's also weird how Hancock was fed that fruit "for fun". The nobles could've had a twisted sense of humor.
- Robin would also be a fun one. She's the "Devil's child", and nothing's known about her father, but she has some unequivocal similarities with Hancock. It's true that a lot of female characters look the same, but the post-timeskip made them weirdly similar. They also have the whole backstory with the government, not wanting to be controlled by someone ever again, and initial bounties of 80m. Robin's mother could've been a means to an end, and that's how came to read Poneglyphs. Ohara were erased from history, just like Xebecs.
- Another one could be Dragon. Garp never stated that he was his father, just that he was Luffy's father, and that Garp was Luffy's grandfather. There's also the fact that Dragon seems quite different from Luffy/Garp. I'm inclined to think that Robin's in love with Dragon, given her reactions to the dragon paintings in Zou, and that'd be interesting, considering Hancock's in love with Luffy, but that'd make for some weird incestuous shit, so perhaps I'll ignore this one.
Lastly, for the mythology enthusiasts, and Oda does draw a lot of inspiration from mythology, there are a bunch of Stars/Planets that are part of the meta plot, and they're associated with gods. Pluton, Neptune, Uranus, Sun, Moon are unequivocal ones.
Venus is the planet usually associated with love, but it's also the planet associated with Lucifer/The Devil.
