Campaign

I think one of the things needed for a campaign is to decide whether
it's operational scale (turns of perhaps a day or a week), or more
strategic (turns of perhaps a month or a quarter, allows enough time
to carry out ground campaigns and build new ships).

For the latter, I was thinking of having three resources: volatiles,
minerals, and power; combine one each of V+M+P to get MCr.   I've
mentioned this on-list before, as I recall.

Part two, though, is to allow players to decide where they want to get
their resources from:
Gas giant (eg volatiles at half price)
Habitable planet (eg volatiles free, power costs double)
Asteroids (eg minerals and power cost half, volatiles cost double)
This way you'd have reason for trade, and reason not to put all your
settlements in one place.