SMT: Lucifer's Call meets GTA IV
So... Like... A really occult heavy post-apocalyptic setting with demons, souls and such.
In a big, rustling and bustling free-roaming city (Tokyo) which got turned inside out into its own inverted spherical world after the conception.
Keep the atmosphere, style and music.
Keep the demi-fiend, the handful of human survivors and the concept of creating/chosing a Reason.
Keep the Magatama to adapt your own skills.
Keep the demon recruiting system, but treat it more like a mobster/gangland thing.
Change the 'demon conversation/negotiation' to something real-time.
Same for the battles, create some gang-like (adaptable?) AI-strategy system.
Add missions, a revised status menu, a more practical/influencable version of Reason creating, with the 'consequence of actions'-concept you see so often these days.
Make sure it's not really fantasy or sci-fi, but really an occult free-roaming warped city at the player's disposal.
damn, I'm gonna have to write this down...