WARNING: The following contains spoilers for One Piece Chapter #1018, "Jimbei vs. Who's-Who," by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
Following the revelation in Chapter #1017 that it was intended for Who's-Who, the latter's battle with Jimbei in the One Piece manga has revealed further important details about the Gomu-Gomu no Mi Devil Fruit in Chapter #1018, “Jimbei vs. Who’s-Who.”
Up to this point, the Gomu-Gomu no Mi wasn't considered to be anything special, other than Luffy’s own cultivation of its power. But thanks to Who’s-Who, we know that not only was the Gomu-Gomu no Mi stolen by Shanks 12 years ago, but he also happens to know about a legend that's theorized to be related to the Devil Fruit’s hidden history. These new details could shape Luffy’s future in One Piece, as well as retroactively make his place in the world even more precarious.
When Who’s-Who was in prison, he learned about the legend of the Sun God Nika. While battling Jimbei in Chapter #1018, he asks the Fish-Man if he's familiar with the tale, as the Sun God Nika was a legendary warrior that vowed to free slaves in the past, but it seems like he never fulfilled his promise. Since Jimbei was the leader of the Sun Pirates, a group of former Fish-Man slaves, Who’s-Who believes that Jimbei has a connection to the story. The fight ends with Jimbei giving off the vibe that he might know more about the legend but isn’t willing to share with Who’s-Who. So the real question is: why does the Sun God Nika even matter in the first place?
While it's just speculation at this point, Luffy’s Gomu-Gomu no Mi could be linked to the Sun God Nika. First off, Who’s-Who seems to be one of the few people to know about the legend of the Sun God Nika. The prison guard he heard the story from disappeared after the story got leaked, and Who’s-Who knew he might be in danger, too. Also, CP0 makes it clear that they are there to verify Who’s-Who death. It seems that he may hold some knowledge that could be a threat to the World Government, which could be to do the legend of the Sun God Nika.
Secondly, if the Gomu-Gomu no Mi was actually the power of a legendary warrior, then that would explain why other pirates were after it, especially Shanks. The Fruit could even have hidden capabilities that Luffy hasn't uncovered yet.
Then there's the theory that the Sun God Nika could also be “Joy Boy.” If the two are indeed related, then Shanks’ connection to the Devil Fruit is even more evident. That would build a direct connection to Gol D. Roger, since he idolized Joy Boy, as well as give Shanks a reason to steal the power that made his former Captain laugh so hard. It could also be who Momonosuke reads about in his father’s diary, but can’t seem to see the connection yet.
If the legend of the Sun God Nika is true, and he somehow also had the Gomu-Gomu no Mi, then Luffy is destined to be the symbol of salvation for many, and therefore, put himself in more danger than he ever knew when he decided to eat the Fruit. It also means that World Government officials are going to hunt Luffy down more aggressively.
Ultimately, if Luffy and Nika are linked, then his final transformation could mirror the Sun God, and that will be when the world realizes who Monkey D. Luffy really is. Already one of the few “Will of D.” users, this could add another layer to Luffy's story that would expand the world of One Piece even more.