What I gathered is, Adventurer class is what we are right now.
If you choose to be any other class, you get specialized bonuses in certain skill fields, but you'll have some skills disabled. Like the Wizard class cannot have the skill Windmill. I also found out all the new skills in G13 are class-restricted.
Down Attack - Knight
Lure of Ballad - Bard
Hailstorm - Wizard
And the new mastery skills are in their respective classes.
You can switch classes at rebirth. Thus, staying/switching classes are optional.
Anyway, it seems like the theoretical hybrid end-game build is no longer with this system in place.