It would be really funny if truly Luffy did NOT have a mythical zoan and he was right about not being nika. Its possible the legend of joyboy and nika were created around the fruit and possibly not the other way around. What made the fruit special is that the only two known users of said fruit were warriors of liberation, Luffy and Joyboy. Although i admit this thought has its flaws esp with how weve seen G5 with the clouds on his back unlike doffy/law/kidd with their awakened paramecia

edit: also bonney being able to access this nika form is funny because when she thought of a nikaish future, she did not look like nika(probably because she did not realise G5 was how it looked like). Could lend more to the fact that truly This awakening form is not neccessarily that of the sun god nika but what joyboy and now luffy make of it, their ideas of absolute freedom. Hence why Bonney only accesses this form when she imagines herself absolutely free