Just to echo what other have said - if you have a game idea, you'll have to make it yourself. I wanted a game where cadets charged naked around a spaceship getting into sexual escapades - not the most common fantasy or scenario - so I learned renpy, Daz 3D and PS and made it. The chances of your particularly fantasy being the thing someone else is willing to spend
years on is... unlikely.
Added to that is the engine you want - whereas a dev can get going with a decent renpy game after a few months planning and character/set design, a 3D sandbox is probably at least a year's full time development before the first - almost empty - v0.1 comes out... and that's assuming that the dev is already an accomplished programmer. The reward for that will be lots of people shouting at you that there's no content, and you wasted their time. It will be difficult to attract patrons (many of whom will fall away given the slow nature of development). It would need a full dev team to provide regular content updates, and probably expensive licensing for the engine and assets.
If you hope to persuade someone else to make this particular game idea, I'd suggest starting out with an initial commission offer of $10,000 per month, guaranteed for a year.
That's obviously insane, so the alternatives would be to 1)make it yourself, 2)substantially scale back your ambition, 3)develop an exciting scenario that hooks a dev's attention.