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I'm a strict defender of the class system. The specialization system, as far as combat goes, started in the Ancient Armies and since then we have the division of "Infantary, Cavalary and Archery - now Artillary". Also, the ability to pick a class accounts for a major choice in your playstyle and how you see your character to start with.
This kind of division makes it possible to enhance each class individually so it gets to the best of its gameplay, on a particular role or playstyle. Furthermore, the holy trinity is instituitionalized for a particular reason: The major aspects of combat in MMOs are: 1) Dealing Damage; 2) Taking Damage; 3) Healing or Avoiding Damage; 4) Controlling Mobs. Unless we invent something such as air combat or marines, I can't see any other big variables coming.
It is true that games such as GW2 are aiming for a remarkable change at this system, but still the paradigm is that in order to grow better in one of those variables you need to lose on the other ones. Therefore, if you're a better damage dealer, you're probably going to be a worse tank, if you're a better healer, you're probably going to be a worse damage dealer than you could be. So that's the central aspect of the class system.
On the other hand, I see a lot of posts about "Freedom in gameplay" and being able to perform sandbox roles such as Crafting in a main way and not simply as some kind of mini-game. So what about introducing some kind of "Sandbox" class? A class that does not stand out in-combat, but rather has its focus on other aspects of the game.
"You know, this place makes me wonder. Which would be worse, to live as a monster or to die as a good man?"