I like the concept! Letting people choose between Solomon's demons is a good idea I think, it gives people a basis for a character and some of those demons are not outright malicious if people don't care to play a seethingly evil demon (despite the "Ultra-Violent Demonic Shonen Battle" title). 

You did mention the option of pursuing the mystery of Satan's disappearance. Would going that route still entail "ultra-violence?" I have no problem with it myself, just curious. I think this has the potential to be a lot of fun - whether people choose to go a solo route and try and rip everyone else apart, or choose more political minded demons forming alliances to gain control of the Throne.