I'm totally in favor of the Advanced class system with unlock quests and stuff.

And for basic classes such conjurer/thau/mara etc.. give some sort of "capstone" (just like DDO).

A skill, or set of skills, or trait, unlockable only at level 50 and usable only when in that specific class. Something "powerful" and meaningful that can lead a player to say : "ok this time i'm gonna be conjurer because i REALLY need that awesome spell/trait".