I'm sure this is very complicated, but I came across the late Imagiscopes Google Earth importer add-on on YouTube, which, unfortunately, I think is abandoned, but it had a very clever feature for importing large areas. This video, which he released over two years ago, shows him setting a focus point and a radius and horizon/edge extent and then utilizing the LOD fall off of Google Earth to import large areas. I have no idea how hard this is to implement, but it would be a really amazing feature that would allow for larger areas to be imported progressively while being reasonably well optimized.