Just like what others are saying, the concept of base classes will only be around for the current jobs. All new classes will be the same from lvl 1 to 60.

(THIS IS THEORY PART) My bet is that they will work like FF11 system. You will be able to use cross-class skills based on your class, but you do not need other classes to unlock the one you are playing. Plus, level does not effect which skills you get. This is my guess.