But what genre should it be in? Sci-Fi futuristic? Gangster? Noir? Steampunk? Isn't the old fantasy swords-and-sorcery thing done to death?
Well, yes and no. Sure, Dungeons and Dragons and Pathfinder are the two big fantasy RPGs currently and there are plenty of other, secondary, fantasy RPGs such as Lone Wolf and Warhammer. But I think there's a reason for that. A generic fantasy world is easily understood. There's very little specialized knowledge necessary to exist in a magical world. Movies such as Conan and Lord of the Rings have popularized the concepts relevant to this genre of games.
For example, in a gangster game what does my character do? How does the mob function? It's much more difficult to conceptualize. Likewise, in a futuristic sci-fi game, understanding the concepts of the world are important. Star Trek, Firefly, Battlestar Galactica, and Star Wars are all tell futuristic stories with space ships but are very different from one another.
I think it's easier for a game master to run a fantasy RPG than other games. Missions can be very simple - travel over there and kill that dragon. Characters are restricted in their options in a fantasy RPG while in a modern RPG characters are much more difficult to direct due to the proliferation of modern technologies. Sure, magic can sometimes be problematic. I guess we'll have to make sure it remains simple in our game.
Besides, there's just something epic about fantasy. So fantasy it is.