I'm pretty sure its designed so that there isn't, that's kind of the point. One character can do it all, HOWEVER subsequent classes tend to be slightly slower to level due to diminishing quests in the world as you use them up and you start to use leves to level further classes, but other than that, the whole intent is to eventually level them all. This is the entire factor of replayability, and furthermore, its required if you plan to min-max due to cross class abilities.