Spoilers for anyone who hasn't gotten up to chapter 967 in the manga
With regards to the iconic Roger Laughing panel when he gets to Laugh Tale, I think the reason Roger laughed when he got to Laugh Tale is because Joy Boy intended his treasure for someone who would live on and lead the world to a new future by overthrowing the World Government.
This would have been extremely ironic, since Roger was terminally ill and had just spent the final days of his life seeking a treasure that was meant for someone else, who would live long enough to be able to change the world.
I think that positions Luffy in a really interesting spot because he has been shedding his life span throughout the manga, like in Impel Down. It doesn't feel like a very Luffy thing to not take the one piece just because he doesn't have very long to live. I wonder if he would take whatever is on laugh tale and through sheer will power force the world to change before dying.
That was really only a thing in Enies Lobby and then Impel Down. I don't think that will be a factor for Luffy. There's just too much child of prophesy stuff riding on him finding the One Piece and being the guy that Joyboy and Roger were waiting for.
As for why Roger laughed, I don't think it was because he was too early and wouldn't be able to fulfil Joyboy's wish. I subscribe to the theory that Luffy's dream is to throw a party for the whole world. The One Piece probably plays into that somehow. Like maybe its some instrument that allows everyone in the world to hear it, or maybe the original Binks Sake or something like that. Something that is goofy and not the traditional treasure that pirates would lay their life down to find. Hence, its funny and that's why Roger wishes he lived in the same time as Joyboy.