What I would like to see is Obsidian making a game based on the PnP World of Darkness - Mage: The Awakening.
If it were up to me, it would feature the cinematic and branching story they are trying to make in alpha protocol, but the gameplay would be equal parts combat and problem solving with Mages complex magic system. It would be a third person game, with pause/real time combat and it would be party based (though a small party, like 3 characters at one time, so as not to burden the engine), and hub based.
The magic system in the game would be unique and try to emulate the one in PnP which to summarize it - allows the player to do anything with an aspect of reality (Forces, Matter, Spirit, Death, Life, Mind, Time etc), if the PC has reached a certain level. It would be extremely challenging to implement (as it would enormously affect the game environment) because the possibilities are limitless ... but not wholly impossible.