Galactic Civ dude. It's almost the same. Only differences are, no micromanaged ground battles, and you can go wherever youw ant in space. Otherwise they're almost exactly the same. Except that Galactic Civ has ship customization, a lot more races to choose from, and less emphasis on the skirmishes.
Anyway, the period of time between the colonies and Halo 2 isn't good for an empire building games. That is, colonizing worlds etc. The emphasis is on the battles not the infastructure. Sure an RTS that deviated from the storyline for hte sake of gameplay would be sort of amusing to play, but Bungie is protective, and the style of RTS definitely won't be on such a grand scale. I'm not saying it's isn't possible, I'm saying I think that there's more potential for badassery in skirmishes and small wars, not drawn out conflicts where it's necessary to colonize and control worlds.