Regarding time vs geography,
If you build some 1944 themed dirt near Normandy, even though the textures and details aren't portable, the actual terrain mesh and elevation data is.
So, if you stitched stuff together on a bigger map, we'd at least have some areas where the actual terrain is high detail that mission makers can take advantage of.
That said, DCS needs bigger and generally just more maps first, and maybe stitching together of the geographically close maps, but I don't think it needs the whole world just yet.