Something with few selections, routes and endings or a proper flowchart like Virtue's Last Reward, I do not like following walkthroughs strictly to get an ending...
As for the story, something sci-fi and/or post-apocalyptic with a few H-scenes that only I would enjoy but would be optional. Lots of adventure and mindfucking plot twists at the end(basically something Uchikoshi.).
I know, I have a limited imagination... I'll probably make another post if I get a better idea.
P.S: Steve, I'd pay to read that...
Also I found your marketing slogan:
"One <protagonist name here> a day, keeps the doctor away!"