Jim, I think it's been fairly obvious to many for quite a while that a 4th race, the one the Vasari have been fleeing from, would be the focus of a Sins2. Ironclad devs have just said they are "not finished" with the Sins franchise as far as confirmation at this point though. The thing I'm curious about is if it would be designed as another diluted SP-MP game features wise, if they would put more meat into both, or if they would just focus on one or the other (in which case it would for sure be SP).

Well... they should better plan on making a SINS 2 someday... or I feel dangerous temptation overcoming me....  

Consider though that whatever was spawned at Kron was enough to collapse the entire Vasari empire.  My current working theory is that it is some race that survives solely in phase space (perhaps as asylum from some ancient enemy that has since died out though they were unaware of this happening).  This was also the race that created Phase Jump Inhibitors, though their original intent was to function as far far longer ranged jump gates than even the Vasari's titan foundry.  The other races simply never figured out how to turn them on properly.

The Exodus fleet is all that is known to have survived (indeed, there are actually probably hundreds of Exodus fleets that received some sort of warning and are all being chased; they just don't know that other fleets exist like their own) unless you include the fragmented survivors of the Dark Fleet.  Consider also that were it not for the Trade Order's economy and just how small the Exodus fleet actually is, the Vasari would have plowed straight through them (which they were doing for the first nine years of the war, before the TEC started producing warships in sufficient quantities to counter the Vasari invaders).  The Vasari at their peak could have annihilated the TEC with zero problems and humanity would become just another enslaved race under the best conditions and extinguished under the worst.

Now think of an opponent so powerful that could bring the entire Vasari empire to its knees.  Such a race would have to be so far advanced that against the TEC or even Advent, their technology may as well be magic.  The Exodus Fleet is fleeing in fear of something that annihilated their primary fleet in one fell swoop with only tiny groups of survivors.  They couldn't stand against this at the height of their power and there is no way they'd be able to stand against them now.

Very few "inventions" of the Vasari didn't exist prior to the Exodus.  Research for them is probably more a matter of decompressing massive files than anything else.  The only new things that race has are a few of the researches of the Rebels: Advanced NME Warheads, Bi-Directional Jamming, Shared Jump Telemetry, and Shared Shield Technology.  None of those things are going to be sufficiently game-changing to make a stand against their chasers.

Even if you achieved some sort of Trinity Alliance between the Vasari Rebels (who don't want to stand and fight by the way, they just want to take the other races with them), TEC Loyalists, and Advent Rebels, you would still have something far weaker than the Vasari Dark Fleet.  The Vasari have hit a technological plateau and the other races are even below them (unless you count the shields of the Advent which are slightly superior to that of the Vasari).  A race capable of obliterating the Dark Fleet would have to be in an entirely different tier of technology.  Maybe if you gave the AR another thousand years to try to find transcendence you would start to see what the new tier would be, but the technology would be so far advanced from anything the races currently have that it would be like the US declaring nuclear war on an ant colony.  It would be so severely imbalanced that one side would always win.

For this reason, I don't know if IC should implement the chasers as a playable or opposing race because you will either have to scrap basic common sense or race balance to do so.

We know that there was one ship that survived and the inhabitants of a planet found the crew driven mad with fear.  I'm sure the Vasari know what they're running from as I'm assuming Vasari have some sort of therapy, memory trawling, or can just turn on a computer screen and look at the ship's sensor readouts from the "battle."  We as players just haven't been told.

If it was something that could be faced, the Dark Fleet would have been victorious unless it had vastly advanced technology.  If it was something biological, the Vasari would have bred it out of themselves.  We know they extensively use nanomachines in medicine and also heavily genetically engineer themselves and subjugated species.  A weakness in technology would have become apparent and building things exactly as they were before the fall would be a terrible idea, yet they do that, as per the VR restoration research.

Any political/other sort of mechanism is also unlikely.  We also know that the Vasari empire never resurged, else signals would have been sent out and the chasers would have stopped which we know isn't the case due to the beacons the Exodus Fleet has been leaving behind.

Someone once calculated the speed of Phase Space (which we can assume the Vasari spend a significant portion of their lives in) to be at minimum approximately the speed of light, possibly being as high as 30c, depending on how long the Vasari generations are and how long the spend at a given planet/system before moving on.

If you are moving anywhere near luminal speeds, you are doing so by technology unless you are a burst of energy at which point the blast would be recognized as such and wouldn't be regarded as pursuers, not to mention that it would have diffused sufficiently over 10,000 years and would have destroyed planets as well as ships, yet some ships survived.

We also know that whatever this is was the Vasari's fault.  The Kron Transport Lab screwed up big time and something they were working on there resulted in the destruction of their race.  All we know is that someone hit the button which caused a massive explosion which destroyed the station and from that, something that destroyed their race was spawned. My guess as mentioned before was some sort of portal that was significant enough for the soon-to-be chasers to notice.

If it isn't an intelligent race, I would like to hear a compelling argument for why it could be anything else.

Maybe that's Sins II

I see what you did there.