Ask any One Piece fan what Monkey D. Luffy's parental situation is, and you'll likely get the same answer: Luffy is the son of Monkey D. Dragon, who is the son of Monkey D. Garp, while Luffy's mother remains shrouded in mystery. This seemingly conventional wisdom has remained true since both senior members of the Monkey family were formally introduced back in the Post Enies Lobby arc, with no further clarity since then.
However, one Reddit user may have found an explanation that turns the entire One Piece world on its head. In their post, u/dfd2002 theorizes that Garp isn't Luffy's paternal grandfather but rather his maternal one, and Dragon's father is someone altogether different -- Rocks D. Xebec, the former most notorious pirate in the world. So how realistic is this theory?
It sounds like an absurd premise at first. After all, isn't it an established fact that Garp is Dragon's father? Looking closer, however, it's never been made explicit in the One Piece manga. Garp is Luffy's grandfather -- the resemblance between the two is a dead giveaway -- while Dragon is Luffy's father, but Garp never claims Dragon is his son. Garp specifically says that Luffy is "the child of my child," which seems a clunky way to phrase it unless you consider that Eiichiro Oda is deliberately avoiding gendered language for the sake of concealing the reveal.
Physically speaking, Dragon is the odd one out among the Monkey family, lacking Garp and Luffy's Cheshire Cat grins and round faces. Instead, Dragon's features are very angular, with a thin, gaunt face and pointy nose. Match that with his long, wild hair and it's a physical description that more closely matches Rocks than Garp.
If u/dfd2002's theory holds true, then Dragon takes after his father in more than just physical ways. Both men oppose the tyranny of the World Government and the Celestial Dragons, although Rocks did so as a bloodthirsty pirate who sought to accumulate as much power as possible. Dragon, meanwhile, seems to do so out of a sense of justice, as leader of the Revolutionary Army. Xebec was the most wanted pirate in his lifetime, while Dragon is the most wanted criminal in his. The idea of father and son leading similar lives and fighting similar battles -- despite having ideologically opposite reasons for doing so -- is a conflict brimming with dramatic potential.
That's all well and good, but his name is still Monkey D. Dragon. Why would he have Garp's family name? As u/dfd2002 lays out in their post, it's not uncommon in Japan for a husband to adopt his wife's family name, in a custom called mukoyōshi. This happens when the wife's family is of more noble standing than the husband's -- or when the husband comes from a shameful background. As the daughter of the Marines' greatest hero, Dragon's wife unquestionably comes from a higher standing than the son of the most infamous pirate. Xebec's legacy was such that the World Government attempted to wipe him from the history books to protect the peace, no doubt painting a target on Rocks D. Dragon's back. It makes perfect sense that he would adopt the Monkey name in an attempt to conceal his identity.
Being the grandson of a villainous pirate adds a new dimension to Luffy's character, too. While Luffy has never let genetics define him, the world pushing expectations based on one's lineage is a repeated theme in One Piece. As Woop Slap ominously ponders in Chapter 96, becoming a great pirate might be Luffy's destiny. It's a seemingly throwaway line -- why would it be Luffy's destiny, as the son of a Revolutionary and grandson of a Marine, to become a pirate? -- unless you consider that Woop Slap knows more than he lets on about Luffy's bloodline.
As for how this reveal would impact the rest of One Piece, the fear of letting Xebec's heir become the Pirate King is sure to incentivize the World Government to pull out all the stops against the Straw Hats during the series' final war. It also adds another layer to Luffy and Blackbeard's inevitable confrontation -- with Blackbeard inheritor of Xebec's will and Luffy inheritor of Xebec's blood, Rocks' legacy is sure to play a huge part in the endgame.
Luffy's parentage has been a mystery for most of the series' run. There's still so much readers don't know about Dragon, and there's next to no information on Luffy's mother. If u/dfd2002's theory holds true, however, there's a very good reason Oda has kept the subject a mystery. Whatever revelations await both the audience and Luffy during One Piece's grand finale, no doubt it will make for some of the most tantalizing drama in the series.