How Does Luffy from One Piece Relate to Nika's Legend?
Nika seems to have been a mythical warrior from ancient times who was venerated as the Sun God by slaves who believed he would liberate them. Nika was first mentioned by Who's-Who in Chapter 1018. He heard about the legend from a prison guard who suddenly vanished after telling the tale. He took this to mean that the information was likely meant to be kept a secret and the World Government didn't want it getting out. Chapter 1044 proved that his thoughts were correct and there's much more to it.
Nika's legend is clearly reminiscent of Luffy's character arc. From the very beginning of One Piece, Luffy has been liberating people from all kinds of suffering. Each member of the Straw Hats was freed from an undesired situation by Luffy whether it was a villain's tyranny, loneliness, imprisonment, impending unjustified execution, etc. Luffy has never been one to stand by and let others suffer. He liberated entire nations from oppression for no greater purpose other than it simply felt right to him.
Luffy with the Mysterious Human-Human Fruit of One Piece
The World Government has apparently been desperately trying to acquire the Human-Human fruit for over 800 years. They claim it was so elusive that it's almost as if the fruit itself was evading them. Initially, this seemed like a joke. However, they go on to casually mention that all Zoan fruits have a will of their own and this one carries the name of a God. It's unclear what exactly they mean by this, but if it's what it sounds like, then it makes sense why Zunesha said Joy Boy has returned in the previous chapter. After all, it can't be a coincidence that Luffy awakened the Human-Human fruit and gained his new Gear 5 form just as Joy Boy has supposedly returned.
The Human-Human fruit grants its user's body the properties of rubber. It's said that they're only limited by their imagination and are the embodiment of the Warrior of Liberation (Nika), putting smiles on faces far and wide. Awakening the fruit greatly increases the strength of their rubber body, giving them more freedom. The Five Elders claimed it's the most ridiculous power in the world. All these details are why the World Government is adamantly trying to eliminate Luffy. There's still plenty of mystery surrounding the Joy Boy situation, the Human-Human fruit, and Zoan fruits in general. Luckily for fans, it seems the time for the mystery to completely unravel, is now.