I'm pretty sure that they've announced that there won't be any further expansions, so don't count on ever seeing a campaign for Sins.  I think the Ironclad people have been quoted as saying that they'd like to do a campaign but don't want to be cheesey or to seem artificial like most campaigns for other RTS games but that they hadn't thought of a way to do that yet.  Hopefully they'll be a campaign or a better yet a custom campaign editor in Sins 2.  What are you hoping to get out of a campaign, anyway that you won't get from the regular game play?  If you play the game a lot and get to the point where you feel good about your knowledge of the game and your skill, and if you're getting bored stomping the AI, you can always log onto Ironclad Online and play against other people in 4 on 4's and 5v5's, at least if you have the Diplomacy expansion.

From an old interview with Blair they said they will make a campaign when they can do it their way. They were not a fan of general MP tutorial theme most RTS use for their campaigns. So you can take that as you will, it may or may not happen.

They said it would be the last micro expansion, though that poll on the page suggests they may be having second thoughts. Or perhaps they mean a full expansion, who really knows.