Prestige Classes for RPG. What do you think?

Instead of having characters start as their class, I've decided to have classes unlocked only by a ~Level 20 quest. Thus characters advance "classless" for the first ~20 character levels, then progress through Prestige Class levels after they manage to complete the secret quests (There will be hints of course).

Some Prestige Classes include:
Paladin
Elementalist Superior (Limited to Robes)
Shadow Assassin
Cleric (Limited to Robes and Leather armor)
Archer
Monk
Bard
Prestige Classes for RPG. What do you think?
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