No campaign ( Devs said maybe in Sins 2 ). What others have done is varied. Some play the Multiplayer scene while others have played the maps and created their own storyline. Some have modded the game as well. The system is designed in sandbox style so you can almost do anything what you want.

Yes, there's no campaign, but the Single Player skirmishes aren't exactly "short missions" either. You can have quite epic games in gigantic star system with many opponents, if you like that.

My approach to Sins of a Solar Empire is similar to my approach to Civilization: I play long games across a few days, and it's quite satisfying.

A campaign would definitely be a nice addition, since the game has such interesting lore. The opening cinematic is so potent that it makes me believe that Ironclad could probably deliver a campaign with fantastic narrative. Would really be awesome for Sins2.