https://www.sinsofasolarempire.com/faq

A: No, Sins of a Solar Empire encourages players to create their own course of events through a sandbox mode. Scenario maps are also included that set the stage for epic conflicts, along with a random map generator. Players can also craft their own maps in-game or by using the included Galaxy Forge tool.

No, because there is no system for building campaigns in Sins. 

It shouldn't be an impossible thing to make maps of the current galaxy layout, like a 3 player medium map where your place is determined by your race (I read somewhere it is not possible due to random placement of teams), but still would be fun to fight in reality, to be Vasari who invades Trader Space, or TEC who defends against these two races in a FFA, and continuing this, making premade maps for entrenchment where you have somewhat fortified frontline worlds and only some uncolonized planets (many planets should be colonized by races at the start), and then Diplomacy map (maybe smaller changes in territories), and last a Rebellion map, a big 6 player map where you can see the actual position of the factions. This would be a good "campaign" in my opinion, and it would only need some premade maps, but I see I am asking way too much from this awesome game. Random maps are good, 90% of the selectable maps (pre-Rebellion) are useless crap, I never liked to play on them, so small place to exist at every map, no real chokepoints to defend well... you take 4 planets and face enemy.. This game should be more than only random maps (although I have played online this winter many times, still would be good to see a way better singleplayer mode as sometimes I don't feel like playing with humans or don't have enough time..)

Sins takes more inspiration from old 4X games like Master of Orion that did not have a campaign (I think most reviews do highlight this fact). What Sins does encourage is for you to make your own story as you play in the sandbox, and for many people this is far more fun than any campaign.

I won't say it is totally impossible to make a campaign, but as a modder I can tell you it would be a lot of work for something that could never be anything like Heart of the Swarm. Still, if you want to get the most out of your purchase I think you're better off trying to enjoy Sins for the game it is rather than fume that it doesn't have a campaign, and they be pleasantly surprised if in fact someone does make a campaign for it.

Actually, if you make a mod specifically for one map, and with clever use of abilities you could get some pretty unique things to happen. The main problem is that we can't display custom in game text to explain a story or guide the player, so an external text file would be necessary for such things.

Games been out since 2008 and no one has created a campaign in that time. Maybe it's possible to do something resembling a campaign. The fact no one has done it proves more telling IMO.

It may simply be the people who make mods just don't care about campaigns. They're already building their own content, surely they'd have no problem making up their own stories in game.

I completely agree with the OP.

I, too, was disappointed with no storyline.  A lot of current games that are not space based have campaigns that people buy JUST to play the storyline.  Being thrust into a galaxy that is arbitrarily at war with greenie meanies and told to "make up your own story" is rather dull.  The game speed kinda gets in the way of a good imaginative story.  For example.

Normal speed game. 

Joe the Trader was having a bad day.  (Trade ship leaves docks.)

Not a horrible day, mind you, but a bad day in most anyone's eyes.  His phase drive engines were barely operational due to lack of payment from his overbearing boss, and he just heard the rumors of galactic turmoil in the sectors he was heading to. (Trade ship at phase jump point.)

Strangely, these rumors of turmoil were inconsistent with what Joe knows to be true.  The galaxy (phase jump underway) had known only peace for all of Joe's life, leading to, up until most recently, prosperity and (phase jump completed, pirates closing on trader ship) a dull existence during his contracts.

Warning klaxons began blaring in the pilot's chamber.  Joe's coffee dropped from his fingers in surprise, spilling molten java into his lap.  Joe sprinted across the the chamber to see what the alarms were.  Phase drive offline already?  (Pirates begin to attack)  Unknown debris found?   

His fingers whirling catlike across the neural interface, Joe (Trader ship explodes.)

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Really.  How can you make your "own story" when all your favorite characters are dead in less than a page?