Suppose a lot will depend on the actual nature of Nika.
If Nika is just a title and a legend that developed around Joy Boy then it’s okay (not amazing or anything, just okay) because Joy Boy‘s been a well established part of the series for a while now.
If Nika was a person on his own, and then Joy Boy had his powers, and then Luffy is the third person in the line, it’s very jarring.
Vegapunk explaining DFs will also be very important to this. Why was a DF made for Nika? Can someone make a Ryuma fruit? He’s been called a god. Where does the rubber come into play? Are we meant to consider Nika a real person, does that mean other mythical zoans like dragons and phoenix and, uh, Buddha, were real in the OP world?
Regarding your last paragraph, an idea I had in mind for SSG's recent invention was that Vegapunk achieved the creation of new devil fruits by giving special properties to animals; for example, he could create a fruit that gives the ability to turn into a dog of magma, something like this. The reveal of the Nika model made me think of the possibility that the ancient kingdom, who was probably advanced in technology, modified real animals in order to give life to their own myths —so something like a phoenix would have never existed but they created it, and such creation was later used for the Tori Tori model.
On a related possibility, another idea "of mine" (because other people may have had it too) is that "devil fruits" were created by the World Government by imprisoning the souls of superhumans (Vinsmoke style) created with the technology of the Great Kingdom; this is why they would be called "devil fruits", because those from the Great Kingdom would be seen as devils (the Ds, in other words). All of it would lead to the final act of liberation, which is returning freedom to the souls in the fruits and erasing this powers forever. If this was the case (doubtful for now because only Zoan models have been stated to have a will of their own), maybe Nika was a legendary figure the Great Kingdom wanted to materialize and things went wrong, or he's the deification of Joy Boy himself as an altered human with rubber powers whose soul was used to create the "Gomu Gomu", or whatever.
Just vague ideas because we can't know right now. The big problem here is:
-If Nika is just the myth created around the figure of Joy Boy, then where does his Hito Hito model come from, why there's a fruit of "Joy Boy's myth" and when was it created since a myth takes time to grow. Maybe someone tried to recreate Joy Boy, is the fruit a way to assure that someone like him comes back in the future because the original was bound to disappear as the mortal man he was? We have a whole century of erased history for these events to happen before the Great Kingdom ultimately falls...
-If Joy Boy had Nika's powers it will be very jarring because it just adds another stage of history that I'm not sure we need. It also takes away part of the interest behind this subplot, because if Joy Boy is inherently related to the "Gomu Gomu" then people like Kaido never had a chance to become the new one, and not because he didn't have the attitude to it or something, but because he didn't have the chance to eat the proper fruit.
So, did Joy Boy have the Hito Hito, model: Nika? If that's the case, when and how did this fruit come to existence? (considering a main subplot of the whole story is artificial devil fruits and how superpowers are created plus the Ancient Kingdom being seemingly technological, I doubt they will be just magic). Or was Joy Boy the same person as Nika, and if that's the case why exists a fruit based on him? Of course they must be related somehow because what Zunesha associates with Joy Boy are the drums that Luffy awakened by awakening the Nika power, so scenarios like Joy Boy and Nika being different figures but converging in Luffy as the definitive person to take the World Government down should be out of question...
I don't know, better wait for more information.
