I've finally been playing Torment... *pauses to allow the outrage and mockery to die down* ...and it's led me to a conclusion, along with Shemeska's storyhour, about an integral element for an excellent Planescape game. I believe it requires a question... a question that drives the campaign. For TNO, it was the esoteric 'What can change the nature of a man?' For Shemeska, it was the much more innocent-seeming 'What is it you want?'
I just thought I would share my revelations for the benefit of all here.
Indeed, such a question as deep as the one you mentioned (or seemingly deep) can truely drive a campaign. A question that truely is about the self of a character, the unique individual, is interesting indeed, and even moreso in a setting where there cannot possibly be a single answer.
Promoting refection of the character on himself/herself is something that should be encouraged in the philosophical-flavoured setting of Planescape. Tying the theme of a campaign to such a question is a great way to do this, indeed (or, at least, it should).
An interesting observation, my fiery friend.
Tell me, have you had the pleasure of finishing the game yet? If so, what do you think about the final answer that is given to Ravel's riddle (the one that appears latest in the game; who gives it, I won't say, in fea of spoiling)?