For years, I have been envisioning the availability of off-the-shelf real-world digital cities. These living, breathing ("real-time 3D") models would have everything the actual cities did, including animated or interactive theme parks, Champ Car or F1 races, AI public transit, enterable actual museums, airports with flight simulation.
3D gaming already offers some amazing 3D world and city editors, especially Auran’s Trainz editor and the new City Life game, but even applications like Roller Coaster Tycoon 3 can be used to create a full city with interactive places and a working culture. Building actual 3D cities using a base 3D engine, and these and other tools, would be a great step forward for reasons far beyond gaming.
I imagine individuals–handicapped people, educators, students, parents, children, the elderly who had previously seen a city but can no longer travel–being able to buy "Atlanta," "Mexico City," "Toronto," "Los Angeles" in a form beyond what has been done in games. They could "visit" places that they would not be able to otherwise, for applications like urban planning, tourism, and other important uses.
Dr Andrew Hudson-Smith seems to be taking similar steps: http://digitalurban.blogspot.com/2006/07/city-life-first-steps-in-render-engine.html. In fact, his entire blog is very valuable.