It's one of the things that I truly want to see....how they'll solve the time paradox of having Yugiri arrive in 2.2 story quest and they even went as far as telling us that she'll be the one to teach us the NIN ways.

The most logical choice would be, you have to get to that point or beyond of the main story for NIN to become available to you...(which of course would suck for new players)
If they want to allow "new" lv30 players becoming NIN while they haven't even beaten Titan NM, I really hope they can come up with a great story or plot twist.....NOT something among the lines of "here, there is this second awesome NIN who's just happen to be here for you to meet".....that's a deux ex machina resource and is usually sign of bad writting.