Posted in a thread but didn't get much attention. But here it is:

A brazilian youtuber that make great theories predicted the JoyBoy's drums thing, besides the obvious thing that Luffy would be Joyboy/Nika too.

In the 'Encyclopedia of things that never were', Joyboy is an entity that smiles and dances, and embodies happines, and etc. It has a drum that plays to heal people from sadness and etc.

Also, he theorized that Roger and Joyboy couldn't be others because there are at least 3 or 4 needs that have to be fullfiled:

Momo as the Moon good (Tsukuyomi), that can control kings of land.

Shirahoshi as the sea/mermaid good (Susano'o), that can control sea kings.

And Luffy is the sun god (amaterasu), that can 'control' people. (not literally, just influence them and bring them to his side, to his cause).

All 3 need to be alive at the same time. Also the Gomu Gomu fruit, and probably the hat. Maybe it has to be an user with D too, but he didn't mention the D in his theory. So that's 3 or 4 check boxes, that haven't been fulfilled in about 800 years.