Maybe kinda like the TES3: Morrowind system, but then spread over the entire game. At the start of Morrowind, you are asked a series of questions, each with 3 possible answers, depending on your answers, the game suggests a class for you. Well, maybe if you spread those questions(and no longer in the form of real questions, but maybe depending on the actual actions you take) you evolve your character into a specific role.
Heh... you mean something like what some games do? Where your choice opens up various "sub-plots" or even "character actions" you can take or even various chapters?
