I plan it more to be a large (but not true planet sized) map for each planet, again something to the size or larger than what you get in ANNO or Industry Giant map or such. So plenty of bases can and will likely need to be established over the map. I imagine you'd play a map like you'd play a city in Sim City or Caesar, just new maps will have different terrain, strategies and quirks. Possibly the map will 'wrap' around it give it a planetary feel, but again it will probably on a reduced scale for purposes of gameplay.
Just a thought, since there is such a rise of popularity in sandbox games, it seems to be the new trend, why not make it a free roaming sandbox? if done cleverly, it might be even easier than scenario based game, you wouldn't need to make all missions and scenarios and stuff, and it sounds more appealing that way.