Worstgen
Hades
My theory is based on the Hindu god Vishnu (https://en.wikipedia.org/wiki/Dashavatara) Hindu mythology. Here it follows a similar pattern with different intent. In dashavatara, Vishnu born in a particular avatar to fulfill a certain task in that era and ends his role.
Hades
Similarly, I think many joyboys have been born over the past 800 years to fulfill a certain thing and pass on to the next generation when they can’t achieve the original goal…like for example, Sun God Nika is a joyboy in one of the timelines and have created a hope for the slaves…I bet these slaves were allied to the ancient kingdom…..
Hades
Roger is the joyboy of his own era and unfortunately he didn’t have enough time to inherit the original will of the ancient kingdom and thus “truly” becoming a pirate king by uniting all the races as "one piece"…so, he started a pirate era hoping someone will fulfill this wish (most probably his son)….
Hades
So, one piece is nothing but uniting all the seas irrespective of the race and live in a single piece and pirate king is the one who rules this kingdom of “one piece”….
Hades
However, Luffy is different kind of pirate king….he won’t rule this seas as a king instead he is a free man who won’t enslave the “will” of these people….