The Stacraft books were okay, I enjoyed them, though not what I had hoped !!!
But I think that Sins would have a better basis and a more interesting plot than any universe I know.
There would be plenty to say about the Advents only by themselves, the begining and their rise...
Same for Vasari with the begining, the apogee and the fall...
The story of TEC would be about chasing the advent and banning them, period of peace and then the discovery of 2 new races never seen before and how they started to deal with them...
Lots of other subjects would be about the war itself and how it forced to the lead to entranchment and in the end, the only solution would be diplomacy.
Story continues, Rebellion, people among their own races are sick of dealing diplomatically with other races, sick of everything it implies, sick of sharing their ressources with them so it leads to a secret group of rebel, growing and growing, and in the end being strong enough to be on their own and fight for their own ideal ! The rebellion would happen more or less at the same time for the 3 races since there would be a first one rebelling and rising and the 2 other races people would just follow example...
Meanwhile, other stories could introduce pirates, how the races use them, try to manipulate them, how they could rise etc.
Story goes beyond game (and plan for another expansion or sins 2), everybody is fighting their own little war and trying to get the advantage over the other but basically, it's going nowhere and everybody stays on the same level... But a threat arise forcing them to ally themselves against it... Introduction of new race coming from unknown part of the galaxy (if an extention was to made about it, it would imply a lot)
Or the idea of the new race could be replaced by: all races are on the same level but the universe is collapsing and stars are strangely dying forcing them to find solutions quickly to "heal" their sun... Many don't and just perish, other succeed to escape... Apocalyptic scenario ending in races working together and being at peace because they saw what fighting together would mean for the survival of their race... or they would, until... TADAAAM *dramatic music* -black screen- TO BE CONTINUED....
Well, all those ideas are pretty basics but would make a great plot if explored deeper with the world of Sins.