Personally, I think saving it for DLC would be the best option for a couple reasons.
1st: I think what's most important for the 1.0 launch of supernova is to launch with all the mechanics extremely refined, bugs as low as possible and all the important information being extremely readable for new players. Granted this is just an assumption, but I am guessing you'll be pushing advertising of some kind after the game launches to attract new and returning players - You NEED that first impression, that first experience to be flawless, especially after some of the "meh" reception of base gal civ 4. So I think you should focus entirely on hammering out, adjusting and refining the important features for now.
2nd: I want a GOOD campaign. As you mentioned, campaigns aren't the normal for sandbox 4x games which means an average or bad one is extremely easy to forget about/gloss over and never think about again. So, if you do a campaign it should be something very special, the kind of thing that takes a large amount of development time/devotion. That's much easier to do if it's own post-launch DLC than trying to shoehorn it in for 1.0.
So overall, I think trying to put the Campaign in for 1.0 would be result in a sub-par, forgettable campaign and potentially impact the quality of the game as a whole which serves neither company nor customer. Saving it and focusing on it as a DLC is the best idea to give players the base game they deserve, an excellent campaign people enjoy playing and of course the best bang for the companies buck.
Just my two cents.