Well, games, broadly speaking, reward depth over breadth, right? In a class-system game, you get the coolest stuff when you pursue one class to the exclusion of all others. (Yeah, maybe you can max out one and then do another, or max out on everything so you're the perfect fighter, mage, and healer, but I believe typically the amount of content in a game is roughly what is expected to get you through maxing out one path.) So I look at Good and Evil as classes, in the kind of game that gets you Good Powers and Evil Powers.

There can definitely be a moral component, and it can be less black and white, but I think it means detaching game morality from this path-based development.

There are a number of characters that have arcs imposed on them by the game, and get no choice about what that arc should be. So, is it possible or advantageous to mechanically encourage some arc, without specifying what that arc should be? I suspect that at least in primitive terms we could do it now, by (handwaving somewhat due to my lack of game-design knowledge) having a series of Turning Point events, after which you reward behaviors that are markedly different from the behaviors before them. I'm not certain, though, whether leaving the moral dimension free-floating is conducive to writing compelling storylines.