In a typical Shonen Manga (or pretty much anywhere, for that matter) there are stronger people that prevent the weaker people from progressing. The hero's crew must overcome these "stronger" people in order to proceed. Naturally, even some of the villains face stronger opponents, and are forced to retreat. These stronger opponents that prevent them from progressing are not always the protagonist and their party.
So, the question here is: What holds certain people back in One Piece (besides the protagonist)? We have some definite answers on some. For example, Moria is in a roadblock because of Kaidou. Krieg reached a roadblock because of Mihawk. But peerless people, like Whitebeard and Mihawk, seemingly have not reached the pinnacle of the Grandline, and none of the other Shichibukai have seemed to reach the New World at all, or at least stayed there for any length of time.
This suggests a total Heirarchy of roadblocks. Going by the typical fighting manga ethos, it's easy to say that because Whitebeard is the strongest person in One Piece, he blocks everyone else from reaching One Piece. But is this really the case? Ace stated he wanted to make Whitebeard Pirate King, but from what little we've seen of Whitebeard, he already considers himself omnipotent. No mention of his desire to be Pirate King was made either way. For that matter, is he really trying to reach Raftel, or caring that anyone else does? His ship, in itself, seems to be floating somewhere in the New World, and it appears that everyone else is the only ones moving. Shanks' ship came to meet his, not the other way around.
If this is the case, what's got HIM blocked from reaching Raftel? What would possibly impede the progress of the World's Strongest Man? We can theorize about things like currents and the poneglyphs and all sorts of other info, but in the twenty-two years following Roger's death, someone would have figured this out, and either allied with Whitebeard to get what he wants or simply using him as a means to an end. Or if Whitebeard's crew had figured it out, then he would already be Pirate King.
Now, if it's a physical barrier (Reef, Seaking, etc, that would not invalidate Whitebeard or Mihawk's title), then another question presents itself: What keeps Whitebeard and Mihawk from overcoming this barrier? If Whitebeard is the world's strongest man, and Roger was equally as powerful, then a physical obstacle would not be a problem for him either. Whitebeard seems equally capable of handling whatever Roger could handle, and Mihawk also seems up to the challenge as the World's greatest swordsman.
Basically, the question is this: What holds certain people back in One Piece? What keeps them from progressing? Is it bad luck, lack of interest, or simply other people who are stronger? And if Luffy encounters the same barriers, what will enable him to overcome these barriers as Roger did?