I think it was Luffy.
Sorry. I mean, I think it was a previous incarnation of Luffy.
I think Joyboy is actually like tons of people we've seen already.. he may have even been Noland at one point.
If Luffy was Joyboy and Joyboy has been reincarnating since ancient times in the Void Century then that means Joyboy has reincarnated about 10 times already and this implies he failed 10 times before to overthrow the government and he also died.
His last incarnation was Roger.
It would bring "new" meaning (I quote new because it would only be new to you guys not to me) to the smile Luffy has all the time and which the D descendants are known for. It's a death smile. They understand they will be reborn and find their friends someday, so they are never lonely and they don't get sad.
Meaning... even if they die- they expect to get a new body and their friends will get new bodies and they will meet each other again because destiny says they will. Also they gravitate toward each other based on liking each other's personalities.
This is basically proven when Roger meets Rayleigh for the first time and he tells him that they are destined to turn the world on its head. Rayleigh had no idea who Roger was back then, but Roger knew because he had previous knowledge about himself.
I think that's probably the problem with Roger.. he grew up knowing about the Void Century and that's what made him fail to become Joyboy. Luffy started his journey without any knowledge of the void Century AND he made a point to everyone at Sabaody that if he found out stuff about the One Piece/Void Century he would stop trying to become a pirate.
That's probably because he has an innate understanding from his previous life as Roger that he can't do that or he will fail like he did the last time. That's why he tempers his curiosity.
I also suspect Luffy doesn't kill anybody (except Kaido) because Roger ruthlessly killed people to get the One Piece, and that turned out to be a mistake. So, he refuses to kill people now.
That one old guy in a filler arc who claimed Roger destroyed his ship and his crew and left him with nothing- that's why he failed. He hurt innocent people and The Heavens decided to stop favoring him.