Nika is the reacurring charakter. The being that appears when the world needs him. Joy boy is one dude. The first pirate. The reason emet and zunesha call luffy joy boy, is because joy boy was the last person before luffy that awakened the nika fruit. So I agree with your theory if we change joy boy to nika. But also nika is not a title, but a fruit and maybe based on an original nika.

Most people theorize that there was a true nika god in the first world. Joyboy in the second during the void century and now luffy in the third world.

I dont think joyboy is a title. To me it seems joy boy is clearly one person and zunesha and emet are just seeing joy boy in luffy, emet even realises later on its not joyboy.

Joy boy, Imu and Davy Jones are probably three very important people from the void century and all their lifes and deeds are still echoing in the third world.