Re:build abolishes the current Rank system and replaces it with a full-fledged Class system.
If we consider the changes, there is little to no difference between the characters that choose the Classes aside of the skills they take, and there are so many resources wasted on e.g. the Class advancement quests and skills developed just to be removed with this update.
I propose to recycle old quest lines provided by Class Masters and old attributes/skills and provide them as additional ways of diversification.
For example, if one has become a Kabbalist, he can now start studying under the Class Master to specialize into one of two or three directions.
The old Class advancement quests will be recycled and allow you to choose one specialization that will provide either a new skill or a new attribute to boost an existing skill (or even 3 options considering there were 3 Class advancement quests for the majority of the Classes in the game).
In the case of Kabbalist,
you could further your studies in the art of Gematria and Notarikon to gain the skill Double Chance (capped at lvl 1, with 50% as attack scaling for the repeated hits).
Or you could further your studies in the theory of the Sephiroth and gain the attribute Ein Sof: Double Magic Circle (will need to be renamed) which increases the OH count on Ein Sof to 2 (so you can use Ein Sof two times per CD now).
If this way of diversification is introduced, there will be even more differences between the different builds of Classes, allowing for specialization, which is part of profession as long as it exists in the human history.