Who's Joy Boy.....
He was promised to pull the noah back to the surface but failed in it hence the poneglyph said it. I believe it will be Luffy, since king neptune is still awaiting for the person to come who will keep the promise. Also as we know Luffy was predicted to destroy fishman island, I think the work of noah is to destroy it so that fishman can live peacefully on the surface without discrimination.
we learnt that like Roger, Luffy has the power to hear things too. I mean he could hear the sea kings but not speak back to them like Shirahoshi could, and the same as on Zou, he could hear the elephant but it was Momonosuke that could talk back to him. It's how Roger was able to read what the Poneglyphs said because he could hear them somehow. Joy Boy could be Luffy but I think that it is too far off in the future to be completely certain. And since Madame's prophecies are never wrong, it means something is going to happen... something big.
But I suppose if anything does happen it will be after Luffy defeats Big Mom and Kaido
On Youtube... theres a channel Joy Boy Theories ... he tells his name's joy boy... so i think its him :/
What do you think?