Ok. It's clear that it would take some time for this idea to be even started. But i would like it to be perfected, so when the day comes it can be implemented without much problems. The idea is, after reaching certain level, a class can specialize and gain certain bonus. Of course to complete a specialisation a hero must complete a quest (maybe put some new NPC's who know more about it). If you have played Lineage 2, you'll know what i mean.
In order to specialize a character will have to do 2 things:
1. Complete a quest.
2. Have an ammount of a certain stat (for example, to became a Dualist the Gladiator will need more Dexterity in order to be able to use Dual Weapons to perfection)
Here are my suggestions, plus some explanations that i got from other games 
Paladin at lvl ?? can specialize in:
1. Crusader -> gains bonus dmg vs. undead
2. Excorsist -> gains bonus dmg vs. demons
Mage at lvl ?? can specialize in:
1. Scholar -> gains identify as a spell
2. Elementalist -> gains a few % to all resistances permanently
Scout at lvl ?? can specialize in:
1. Tracker -> gains permanent Infravision
2. Hunter -> gains bonus dmg vs. creatures
Gladiator at lvl ?? can specialize in:
1. Dualist -> Gains bonus to ??? when wielding two weapons.
The Dualist is a nimble, inteligent fighter trained in making precise attacks with a weapon in each hand. He always takes full advantage of his quick reflexes and wits in a fight. Rather than wearing bulky armor, a dualist feels the best way to protect himself is not to get hit at all.
2. Berserker -> Gains Improved Fury
The madness and unpredictability of a thunderstorm is the soul of the berserker. Unlike most other characters, he does not fight to achieve some heroic goal or defeat a loathsome villain. Those are mere excuses - it is the thrill of combat that draws him. For the berserker, the insanity of battle is much like an addictive drug - he must constantly seek out more conflict to feed his craving for battle.
Assasin at lvl ?? can specialize in:
Now i think this class can be renamed to rogue, as in original Diablo, and then become an Assasin.
1. Trickster -> ???
Tricksters combine some knowledge in magic with a taste for intrigue, larceny, or just plain mischief. They are among the most adaptable of adventurers. Rather than plain confrontation, they like "dirty fighting".
2. Shadowdancer -> ???
Operating in the border between light and darkness, shadowdancers are nimble artists of deception. They are mysterious and unknown, never completely trusted but always inducing wonder when met.
Monk at lvl ?? can specialize in:
1. Monk of the Sun -> ???
2. Monk of the Moon -> ???
