Well other than the two classes we know will be added next? ( Arcanist and Musketeer) Also some may mention DRK or other such classes, in an interview on one of the fansites it mentioned a system being worked on , here I will post this one question from the Eorzeapedia.com interview.
Q. You have also mentioned the possible introduction of traditional names. How do you see these new names fitting into the existing class system? Do you plan on using the new names as a guide to develop class uniqueness?
A. Yoshida-san is between two possible plans at this time. The first idea is that the traditional names would be introduced as “higher” or “advanced” classes. The traditional classes would thus be something of a goal for players. The second idea is that the traditional names would be more of a subset or specialization within the existing classes.
Whatever change is put into effect needs to be right. He recognizes that he will not be able to change the system again after this, so whatever is put into place needs to work, and be able to be balanced, for years to come. Because of this his team is giving this issue a lot of thought prior to implementation.
New classes I would like see is Bard or other buff/support classes.