TFTUV has a futanari character and my next game will likely have something similar as well. But doing a whole game featuring such a specific niche would alienate a lot of the existing player base I'm afraid.
True, it really is more work to have to develop characters, stories, personalities, how you will conquer and how the story will follow, I wish you luck and success for that. But then you already know the 3 topics most desired by your audience: harem, incest and pregnancy, why don't you ask if the audience would like some more? If there are more than 1 you choose at least 2 to add to these topics.