That is all really interesting speculation, but I'm not describing a system which could be, but one which is already available and fielded. In cruise missiles it is called DSMAC.
Basically inertia guidance enhanced by terrain matching. Which is great, but terrain matching as a stand-alonenis pretty useless. And it still requires good map data. Fine for a cruise missile launched from a base or ship. Becomes an operational issue for cheap throw-away drones launched from the middle of nowhere.