What do you think a campaign would add to the game other than cheesy cutscenes and a hackneyed story?  If you want some good science fiction stories, I recommend space movies and sci-fi novels.

The game is about what the game is about--expanding your empire, making the right investments, putting together a good fleet, managing battles, and making good decisions.  I really don't see how a campaign would add to the game play.  If you want you could play through each map sequentially and then download custom maps and mods.  (If you get really bored with the AI and can mop it up, you could also try the game in online multiplayer, playing humans v. AI comp stomps and also against human opponents.)

All that having been said, I agree that it would be nice if the game had a campaign simply because many people have requested it.  I guess the easy way to do it would be to brainstorm a silly story and then have cheesy cutscenes interspersed with a sequential playing of the standard maps.  Winning the battle on such and such a map would be made to tie into the story somehow.

I suspect a campaign of some sorts will be in the next expansion/sequel or whatever will come after Diplomacy. Keep in mind that this game was a surprise hit, so it is really not that surprising that it so far hasn't had one. But now that Ironclad has some experience and more importantly a very sucsessful commercial record, I think they'll start to upscale their ambitions.

I would enjoy a campaign to the main game, an official one mind you, but I have to agree with -Ue_Carbon, each game is sorta like a campaign.  One could imagine a sequence of events around each one, make up historical events, heroes, villains, et cetera.  Like the old saying goes, "Just use your imagination.".