I would love a game that addresses all of my favorite styles. Have an RPG that you can roam all over, link up online (if you want, but totally not necessary) to find new realms, visit other realms/worlds that are specific to various things (trading, gambling like Vegas, hunting, sims, etc.) These worlds would enforce the rules special to it (no fighting in the casinos without consequences). There could be duels similar to the fighting games as if in an arena. And you wouldn't have to master them all to beat the game or have fun. You could simply fight, do the rpg part, play the arcade/casino games, build a city for others to roam/gamble/fight in. You could completely control the look of your character and then they would evolve as you make decisions, use certain weapons, or learn certain powers.
You could be a tycoon and grow your city, you could live in a city and be a Sim, fight baddies in FPS and be like the Master Chief, defend the weak and become famous, crush the weak and become notorious, buy weapons or stuff, make weapons or stuff. Marry another character and have a kid that is a combination of the two of you (that would be different, but it could work).
SO... roll Knights of the Old Republic, Sim City, Warcraft, WoW, the Tycoon games, arcade games, casino games (like video poker), Fable, Battlefield 2, and Halo into one big package. It would probably take a few BluRay discs to work and one hefty computer. One disc per world?