Well, like all gamers I have a lot of differant ideas that I think would make for awesome ideas. None of which would fly of course, for reasons people have pointed out time and time again. I think The Escapist did an article called "your game idea sucks" or something like that, which pointed out the sad realities of the game industry in this regards.
As much as I love RPGs, and MMORPGs, chances are if I ever became a billionaire and was to helm the production of a game, I would probably sit down and try and create a sandbox horror game. I've mentioned this before. The idea occured to me with all of these sandbox crime games where people run around doing horrible things, and terrorizing the civilian population. I mean in Grand Theft Auto, a common way to make money is to hide in alleyways and kill passers by (especially hookers) with knives, in hopes that you'll get one that will drop a couple hundred dollars.
I really liked 1980s/early 1990s horror movies, where we had iconic franchises like "Friday The 13th" and "Nightmare On Elm Street" to name the most enduring ones, not to mention numerous stand alone productions. This lead to people sitting there wondering who would win a battle between their favorite horror movie bad guys... something which even eventually inspired a "Freddy Vs. Jason movie".
I sort of had this vision of a sort of sandbox action/RPG set in a city where you could create your own horror-movie type bad guy, and pretty much go on a rampage of "adult level" rape, torture, murder and destruction, and ultimatly wind up fighting other horror-movie type baddies/rivals for territory and the like. I was also looking at the way police respond in games like "Grand Theft Auto" and was thinking that it could be interesting to plot police response patterns based on things like the ancient "VICAPS" (Violent Criminal Apprehension System). Things like the "least effort" principle so a budding monster would for example have more problems with the police if they took victims too close to their lair/home, especially to begin with and things like that.
It's complicated, and doubtlessly a bit sick, especially if I went into the overall plotline/missions/etc... which I've vaguely considered. However I see it as being in a similar spirit to the "giant monster" games that were popular for a while.
How well it would do is beyond me, but really I've never run into anyone else that has had similar ideas. Also to be honest it's one of those things I don't think would make a fortune, but would probably have a healthy niche market and be somewhat profitable. Chances are if I ever did it, I would already have money by definition, and I'd be making the game for horror fans more than any real craving to get even richer off of it.