Of all the different running stories in One Piece, there are two storylines that hold utmost importance – Getting to Raftel (Pirate King), and the Void Century/Forgotten Kingdom. Why? Because Raftel is sealed off, and it relates directly to the Void Century.
Now, the above was a bit of speculation, but it's educated speculation. Consider for a moment what we've observed in the storyline so far -
Whitebeard wants to be Pirate King (as noted by Ace's words), and hasn't been able to achieve it in all this time.
Shanks and others were in Roger's crew and none of them have been able to get back to Raftel in the time after Roger.
These points of information lend themselves to one critical conclusion - There is a power or force that has been sealing Raftel away, and only Roger was able to figure out how to bypass it.
I'd like you to consider what we know about Roger now. In addition to being a powerful pirate, he had two other key attributes that likely had a direct impact on him being able to break Raftel's seal.
Roger actively pursued knowledge of the Void Century.
Roger was a conduit for the Will of D. (Gol D. Roger)
In essence, these are the two attributes Roger had that seperates him from other strong pirates capable of sailing the entire new world. So all of that being said, what am I presenting to you in this thread?
Just this hypothesis -
Raftel is currently sealed, and the power of which is such that no one is able to break it. The key to entering Raftel rests with the Void Century/Forgotten Kingdom, and those who carry the Will of D.
Hard to speculate much further than that, but it does seem like a direction that Oda may take to tie together some of the different plot lines he's been introducing (as well as filling certain plot holes)