I would be very in favor of a good campaign, but I don't think that the setting should be with the Vasari and the darkness, because of (among other things) balance issues and believability issues.

Remember that the Vasari tried to fight them a long time ago by sending their whole fleet to fight the darkness but only 1 ship returned or something, so they packed up and started fleeing. If the campaign had something to do with this it'd kind of be pointless because the outcome is completely predetermined. It'd also be depressing. If the campaign would be about how now, while attacking Trader and Advent space, the Vasari suddenly decide to turn around and fight, that would make very little tactical sense.

The "Lore" tab says this:

"Now, the Vasari are caught in a stalemate, and in some positions, on the verge of being pushed back. A state of panic ensues. Already too entrenched in this disastrous war, the Vasari are simply unable to withdraw without catastrophic losses. Even worse, the conflict is consuming resources faster than can be put into the reserve. They will not be able to fuel the next phase of their exodus and time is running out."

If they don't even have the resources to keep traveling, then their now-much-diminished Exodus fleet is certainly no match for whatever's following them (the Dark Fleet that the invaders obliterated was much more powerful than the original Exodus fleet, and the current Exodus fleet is weakened from fighting the Advent and TEC)

If, on the other hand, the campaign was made to be winnable, it would almost certainly be done by most players by using stupid exploits of the AI, such as suiciding themselves on your starbases, or by jumping in and out of their gravity wells repeatedly so they can't advance... etc. I don't like the thought of campaign-players saying to themselves, "Hm, this invading force was supposed to be unstoppable, why are they making such stupid AI mistakes?"

As it is now, the current plot for Sins of a Solar Empire is basically an excuse plot for watching ships blow each other up. Not that that's a bad thing.

This said, I would be in favor of a good campaign addition to Sins so as to help teach newbies better strategy, and hopefully to encourage them to try multiplayer (in addition to lowering the learning curve required for multiplayer).

Like Wrath said, you would tend to think that whatever race is chasing the Vasari would just roll over the less technologically advanced Advent and TEC.  If they do ever introduce a 4th race, maybe they could call that expansion "Invasion".

Yea true, I spose it could somehow be a ratio between Defense teams starting worlds (including a score for different planet types) and Invader teams number (in supply) of ships. Would definitely be difficult to work it out in a balanced way however. And chokepoints would play havoc on the balance as well.

Would be great fun tho. It would be great fun and would take some good knowledge of the game to work out which world to fall back to on the defense side. On the over side of the coin, the Invaders would need to fight very carefully with their limited ships. I can picture some incredible Advent plays in the hands of a skilled player as an Invader.