When I was 5 years old, I started thinking about this. I wanted to make my own Doom, after a few years I wanted to make my own Quake 3 Arena or Unreal Tournament. After more years as I learned the things around making games I would be the happiest person alive if I could make either sci-fi FPS or some kind of 3D RPG possibly something like Final Fantasy games at PS2.

Lords of the Realm management with Mount & Blade combats....

Thanks for all the replies guys, mine would be a GTA times 1000, with all of USA explorable, every building enterable, realistic graphics.