It seems like a good idea to me. There is still a long way to go to think about the part of the sex system. For example, I have the combat system moderately thinking and it will be inspired by "sengoku rance" and that would be coded first. But if you want to collaborate designing the sex system would be excellent.
I've never worked in Unity, and as a coder I'm kind of lazy (when I'm trying to do something new, sometimes I know there's a better way, but...). I'd love to contribute, make suggestions, and help out generally, but you'll need someone better at code to lay a solid, efficient groundwork and fully implement. Once a framework is largely in place (for whichever system) and adding content is mostly copy/paste/edit, I'd be pretty handy for fleshing out extensive branching and procedurally generated content. I can't get enough of games where every playthrough is different. :]