i suggest if there is prestige classes game will be more pro game and more interesting cuz now all characters are similar but balens users and this a little bit boring, we will need more skills to do what i am suggesting and according to skill distribution we gain the prestige class with a quest and we gain some spaciel abilities mostly passive ones, here are some suggestions for classes

knight;
-divine champion
-sword master
-berserker
-vanguard

mage;
-archmage
-fire mage
-wild mage
-holly mage

archer;
-arcane archer
-duelist
-assasin
-shadow dancer

as we distribute our skills we will gain one of those prestige classes, the point is there is no lvl limit for this only skill combination, and bonuses can be extra life, def, attack, crit rate, crit dmg, rage bonus...