I could go either way with skill-trees or class systems, but a lot of it boils down to the way the world is portrayed. If the game world is more egalitarian and there's a lot of bleed over across roles in society, then having fuzzier distinctions that skill based systems provide (like Fallout and Arcanum for instance) could work but if we're talking about rigid caste systems where fighters fight, thieves thieve, Sorcerers ensorcel, then it seems like a class based system is the way to go.
My guess is that they'll probably go with a class system of some sort with class specific skill trees nested within those classes for differentiation.