If I were to do one (In the future)
I would follow the central line of SMT
Kinda putting an in-apocalypse or post apocalyptic context that represent one of teh many battles between Law and Chaos
Players would be forced to chose an initial alignment (the one can change at the course of the RP), And an initial demon (I would add demon's restrictions according to Alignment. Kinda a Chaos cannot use an Archangel for example).
Of course it would be more story centered. But for that I need to develop a good plot.
The other problem is that I'm already weaving an RPG, and have several ideas for the next ones. So I don't know what I'll pull first.