Jusst an opinion here, but i think the "expansion" framework might actually be the way to tell the story, and lay out the campaign, we start with Sins vanilla 1.0 and then move up to upgrades on ships as the war between the Tec, Advent and Vasari get under way, we move incrementally up with tweaks in the ships, in abilities, to the point where we actually begin to "Entrech". Historically on the timeline, this is ten years after the war has started.
Tec and Advent and Vasari have been at this a long time now, with heavy losses and heavy changes in technology, each side now has starbases and heavy anti-structure ships.
"Diplomacy" is coming, and part of the tag says "twenty five years after the war has started, which of course moves us further down the historical timeline. With diplomatic liasons and envoy craft.
Beyond these, is something on the horizon that you or i dont even see yet, but the devs do, and maybe this is how they will do it, not an "all at once" maga campaign expansion, but incrementally, getting there in say, what do they want or forsee? Diplomacy and then Sins2? Or Sins2 and three full sized expansions beyond that? Like WOW or GuildWars?
We already have battle coming out our ears, and so we move by degrees toward other facets of the game, to make it broader and more interesting on different levels. So now we can play the same game in a variety of ways, with different strengths and different emphases.
Maybe down the line, they will hit us with an "In Your Face" all out massive battle assault, where we are almost outstripped in tech and ability and have to hang on with our fingernails to get through a game?
Haha, might be cool, very very cool in fact to go up against something that blows everything i can do at it almost away.
Haha,
Instead of freaking out and not playing i would probably figure out a way to do something, 
Anyway, my point is ... It might be better to go incremental and tell a broader and bigger and larger story, rather than trying to cram everything into one budget and one expansion to try and fullfill everyone's expectations of what a Sins campaign should be.

-Teal