So, I believe it works like this.
Nika: slave who rose up in Ancient Times, and defeated an Ancient Enemy. Had the same properties as Luffy (but with no Fruit)
-Nika helped pave the way for the Ancient Kingdom to be developed. The Tree of Knowledge in 5,000 years old, and the Alabasta palace in Alubarana is 4,000 years old
Joy Boy: the first pirate who grew up during the Void Century, and lost to an Ancient Enemy. Most likely had a Devil Fruit. Was friends with the Princess of Fish-Man Island.
-Joy Boy lived during the collapse of the Ancient Kingdom, where their technology as powers led to them becoming too powerful/arrogant, and then being destroyed by those own powers (Ancient Weapons, Mother Flame, Devil Fruits, etc). Joy Boy tried to stop this collapse, and became a pirate to try to take things back from the Ancient Kingdom, but, in the end, he and his crew were betrayed by other allies who joined the WG instead, wanting its power, rather than freedom. This is why his crew is constantly apologizing; his crew sided with the WG/Ancient Kingdom over Joy Boy, and, as a result of trusting them, they all lost.
So, Nika helped create the Golden Age of Technology, Joy Boy watched it be destroyed, and Luffy is here. That's really cool if Oda is going with that.
And ya'll just care about ninja aliens and rubber Jesus.