So heres the basic concept you play as a demon think Viper GTS. You are summoned to he human realm, but you don't want to go back.
I have decided on two major mechanics. The firs is the magic circle. The circle will be a portal between the mortal and demon realms. I am expanding on Sean Howard's - Demon World mechanic for this, so in his example if we think of the magic circle as a base and it's radius is proportional to it's power. Now i thought it sounded unfair that people can go whenever thay wanted... unless by stepping outside the barrier it just puts them in the demon realm and to get back the just need to walk back to the circle. In this way it's like a window not a door.
The second is a dating simulation... well I'm thinking more like Harvest Moon BTN. Mixed with a character creator. Instead of trying to find out if someone likes apples your trying to find there sexual desires. You will be able to change your appearance, and gender in order to align with there fanticy to seduce them. Once seduced you can feed off there life force to increase your power, or suck them dry and take there soul to expand your magic circle.