Nika is not Joy Boy & Luffy's DF is not Joy Boy's DF
Only thing that made people say Nika = Joy Boy is Zunisha's Comment about Drums of Liberation
But One Piece Story is about Inherited Will, all SHs are related to Joy Boy, all Ten
Think of Joy Boy as Oden, Luffy as Momo & Nine SH as Nine Scabbards
They all Inherited Oden's Will & their Haki feels like his
Chopper is literally from Drum Island & go check how Oda drew him when he was first mentioned
Go check how Zoro started talking about his Heartbeat & Heartbeat of things around him when he faced Mr. 1
Usopp's Epithet is God, he saved all Slaves of Dressrosa & he was the one drawn with Drums in Skypiear Party
Brook is the one associated with "Laugh", go check how he was drawn against Fishmen Post-TS & he is who returned from the Dead
As Kaidou theorized Luffy is Joy Boy, the latter declared his Ideal World & it matches Sanji's Dream & what he stands for
Oda made Jinbe's Former Crew called "Sun Pirates" & WsW theorized that Jinbe is somehow related to Nika & he shared his Blood with Luffy
Nami have so many Sunflower References
... etc
Oda answered who is Joy Boy already in Chapter 1
When Higuma humiliated Shanks, how did Shanks react? He Laughed, what happened after that? His Crew Laughed with him
How did Luffy react? He got angry & questioned why are they laughing in such a moment!
Shanks is Joy Boy 2.0 just like Momo is Oden 2.0
And he also have Nine Crewmembers/Commanders
However Shanks Crew aren't the ones who inherited Joy Boy's Will, they are not like Scabbards who inherited Oden's
Hence why Oda made Luffy make a promise to him "I will gather a Crew GREATER than yours"
This Promise isn't just for Power Level Purposes, it's Oda indirectly telling us that you need Nine Shadows to fulfill your destiny
And Luffy is the one who's gonna accomplish what Shanks failed at.