Maybe think a bit smaller, perhaps? A Forza that allows you to go anywhere in the USA with real life length roads just like Midtown Madness 2's Cruise mode with no policemen and Flight Simulator, but with Cars/Trucks/CUVs/SUVs etc. Combined that is also open world perhaps?
Even America is ridiculously unrealistic, even a single state is. way to much data, and collecting the data to the details required would take decades and after that it would just look like the Google street view.
Until you have super powerful AIs to convert satellite images to real high res textures and ground models and scenery, it onto ever happen.