WARNING: The following contains spoilers for One Piece Chapter #1018, "Jimbei vs. Who's-Who," by Eiichiro Oda, available in English via Manga Plus and Viz Media.
On top of the brutal fight between Jimbei and Who's-Who, Chapter #1018 of One Piece also revealed some interesting tidbits about an old legend. While imprisoned in Impel Down, Who's-Who was told of Nika the Sun God, a beacon of hope that kept his spirit alive through daily torture. Though Who's-Who isn't the most credible source, there's a bit more to the Sun God legend than meets the eye, including a possible connection to Luffy.
According to Who's-Who, in the past, Nika was prayed to by enslaved people who believed he'd free them from their oppression someday. It's unclear if he was actually real or not, or how far this myth goes back -- it could even be from the Void Century, there's so little to go on. However, the reason Who's-Who tells this to Jimbei is that Jimbei's the former captain of the Sun Pirates, a crew consisting entirely of Fish-Men, with a majority having been former slaves of the Celestial Dragons.
Fisher Tiger took it upon himself to free the people under subjugation in Marijoa after being a victim of slavery himself. His actions saved countless innocents, including Boa Hancock, her sisters and other Fish-Men who eventually join him in starting the Sun Pirates. To hide their former enslaved status, every member of the Sun Pirates covered their Celestial Dragon hoof branding with a brand of the Sun, deepening the bond between them. As a hero who became a beacon of hope for enslaved people with a sun theme, Nika and Fisher Tiger share this in common.
There's also further evidence that this isn't the first we've heard of Nika. In Chapter #287, the people of ancient Jaya are suffering from a plague. The Jaya elder proclaims that the most beautiful maiden must be sacrificed to their god in order to save their village. Kalgara's daughter, Mousse, is chosen and, though her mother is devastated, Mousse seems calm about "going to meet the sun god." This flashback takes place 400 years before the series' present, which aligns with Who's Who's statement about people "in the past" praying to Nika. This also isn't the only moment in the Skypeia arc that connects to Nika, as two moments pose a possible connection between Luffy and the Sun God, as well.
One of the Skypiea arc's most memorable scenes comes in Chapter #253 as the Straw Hats have a bonfire party with some wild wolves the night before they go search the city of gold, Shandora. In a charming two-page spread, as Robin watches everyone dance around the fire, their silhouettes perfectly show which character is which. Strangely though, Luffy's pose in this spread is nearly identical to the silhouette of Nika on Chapter #1018.
There's a similar scene in Chapter #300, during the party between the Skypieans and the Shandians. Here, Wyper wakes up, taking in the scene, and there's a small panel focusing on the silhouetted Luffy -- and just Luffy -- as he's dancing. With this being the second time Luffy's appeared in a soli-black, Nika-like image, it seems more deliberate than coincidence. The look of Luffy dancing by the fireside seems to suggest that our favorite rubber man is linked to Nika's legend in some way.
Despite this, it's unlikely that Luffy has secretly been a god this entire time. Similar to the "Luffy is a reincarnation of Joyboy" fan theory, it's more likely Luffy could be in a reincarnation of Nika in the same line. Luffy sharing this Sun-theme would also thematically pit him against Blackbeard, given that the dread pirate shrouds himself in darkness with his Devil Fruit power.
Chapter #1018 of One Piece subtly leaves us with a lot more to think about. One thing's for certain: this won't be the last we hear of Nika, and in the direction Wano's heading, the story's likely ramping up for a big reveal before the end of the arc.