*Spoiler alert, contains information from the latest Manga Chapter, 653*
I have a theory. The compass doesn't lock on Raftel. I just thought about it. What if when some pirate crew miraculously tries to reach the final island, they instead find the end of the grand line back to one of the Blues? It's like, they think it's the final island, go there, and then suddenly they're out of the grandline. To reach the End of the Grandline, one had to go a different path.
Now that I think about it, it's a bit weird that they showed us a compass that determines which island is the calmest. I know it requires a great navigator to understand which one is the best (Though I kinda looks it's more like, requires a person that knows when something shakes more xd), but still, doesn't that seem way to easy? If they hadn't left FI, I'd say next chapter someone was going to warn them that the more easy islands you pick, the higher the chances of you falling out of the Grand Line and go to the Blues. So it's like, you must take some risks, or you will loose the "game". But since they were in a rush to leave, and the King didn't want to bother them with bad news, maybe they hid that information, and someone later in the game will tell them.
Following that line, I'd say that if you always fallow the most dangerous island, eventually you will reach Raftel, whereas any other course would take you to the Blues eventually. Luffy already wants to do this, because of the adventure, and he thinks a lot like Roger. Maybe he too followed the most dangerous course, when no one else dared to, and thus he reached the end of the Grand Line.
Even if my theory isn't right, now that we are presented with option of route, we can try to find a key. If we see that they keep choosing the hardest route and find signs of Roger, then maybe my theory is correct.
Another theory is that they find phoneglyphs and the phoneglyphs themselves hint which route to take. The first clue should be on Fishman Island's phoneglyph, and thus I'd expect Robin to say anything in the next or so chapter regarding that, and confirming the theory, but in a sense, I think it would be too simple if they discover the key that soon.
I have a theory. The compass doesn't lock on Raftel. I just thought about it. What if when some pirate crew miraculously tries to reach the final island, they instead find the end of the grand line back to one of the Blues? It's like, they think it's the final island, go there, and then suddenly they're out of the grandline. To reach the End of the Grandline, one had to go a different path.
Now that I think about it, it's a bit weird that they showed us a compass that determines which island is the calmest. I know it requires a great navigator to understand which one is the best (Though I kinda looks it's more like, requires a person that knows when something shakes more xd), but still, doesn't that seem way to easy? If they hadn't left FI, I'd say next chapter someone was going to warn them that the more easy islands you pick, the higher the chances of you falling out of the Grand Line and go to the Blues. So it's like, you must take some risks, or you will loose the "game". But since they were in a rush to leave, and the King didn't want to bother them with bad news, maybe they hid that information, and someone later in the game will tell them.
Following that line, I'd say that if you always fallow the most dangerous island, eventually you will reach Raftel, whereas any other course would take you to the Blues eventually. Luffy already wants to do this, because of the adventure, and he thinks a lot like Roger. Maybe he too followed the most dangerous course, when no one else dared to, and thus he reached the end of the Grand Line.
Even if my theory isn't right, now that we are presented with option of route, we can try to find a key. If we see that they keep choosing the hardest route and find signs of Roger, then maybe my theory is correct.
Another theory is that they find phoneglyphs and the phoneglyphs themselves hint which route to take. The first clue should be on Fishman Island's phoneglyph, and thus I'd expect Robin to say anything in the next or so chapter regarding that, and confirming the theory, but in a sense, I think it would be too simple if they discover the key that soon.