Isn't planetary annihilation exactly the type of game you're looking for? While I don't think it's as good as supreme commander (it gets too complicated to operate stuff in late game IMO), but it executes the premise of using full surface of planet (and multiple planets in fact!) for the campaigns.
The planets in Planetary Annihilation always feel too small to me. 80x80 maps in SupCom feel bigger to me than a series of (what feels like) golf balls.
I've played it, and it does some neat things (spherical battlefields lead to interesting tactical decisions).
But the idea I was thinking on was closer to, when Command and Conquer has you advancing across Europe...I want the map to just keep scrolling at that scale. Let me finish a mission by building an MCV and having it just drive way off the map.
