Doesn't even have to be accurate, although it probably needs to look realistic. Like I really only need the aesthetics, not the specific placement / distance of buildings. As long as I get the "feel" of familiar cities / nature that would be awesome.
Geoguessr is already a popular game, and all you do is travel around a location and guess where it is.
Latest FlightGear already supports OpenStreetMap's OSM Data to be used as scenery source.
> Like an FPS or racing or sim game that lets me play in my favorite cities, or better yet, my favorite towns.
In most cases you could convert OSM Data of your town using Blender into scenery for such games, which supports import user created scenery. May be some additional export add-ons for Blender would be required to instal for specific game file formats.