We know the world is round. Also, if you look at where the red and grand line cross on a map, with each of the other four seas in their respective cardinal directions, then the red and grand line form an X. From the whole classic "X marks the spot" pirate quote, I used to have a theory that the one piece (and Raftel) is actually inside the red line, likely at reverse mountain.
This would explain why no one has been able to find it, and it would also follow a little theme I've noticed about "impossible" islands in the grand line (ie Sky Island = island in the sky; Fishman Island = island under the sea). The next "impossible" island I saw in this trend would be an island within land itself (Islandception).
Due to recent events in the manga/anime however, this theory is probably busted.
This being said, in the interest of speculation, if I were to try and find a place to enter Raftel if it were truly inside the red line, I would first look at how the grand line was first entered. When the Straw hats entered the grand line, there was a current which would pull them up a mountain, but if they weren't exactly in the center of the current, it would pull them under water. There could be a special spot where they would need to get their ship pulled under the water, where the current would take them through an underwater cave or something, and spit them back out inside the red line.This explanation would also give a good explanation for the all blue, being that it sucks immense amounts of water in from each of the four seas and ejects them out into a single body of water.