I think reaching Raftel requires a combination of poneglyph reading ability and very high navigational skills. Now, poneglyph reading part is very obvious and Robin's importance in that regard has been overtly conveyed by Oda in the manga. I think that Nami's role is equally important and no other Straw Hat has greater role to play. My logic is that Raftel is probably in a sea-maze within the most turbulent and unpredictable part of the New World, and only those who are able to guide their ships through that tumultuous maze can reach Raftel. Thus having a map is not enough, someone who can navigate through the horrible whether and ocean patterns is needed i.e. Nami (her lack of knowledge in certain fields of navigation is not important, because nobody has any knowledge for the regions near Raftel anyway). Now, I think that Blackbeard and Shanks will figure this out and understand that among all pirates only Luffy can reach Raftel due to having these two. Perhaps this will provoke Blackbeard to engage Luffy and take them to reach Raftel. Or it could provoke Shanks to confront Luffy in order to kill them (if he doesn't want One Piece to be found). What do you guys think. Do you think Nami and Robin will more highlighted in future arcs near Raftel?