You all forget about the "Unknown Enemy" that the Vasari are running from. This is pure speculation, but i think Sins 2 will be based on the "Unknown Enemy" that the Vasari have been running from. They will finally arrive, and cause chaos in the known Tec, Advent, and Vasari Remnant occupied regions.
Sins, Entrenchment, and Diplomacy set the stage for (picture star control 2's spathi voice) "The Ultimate Evil" to arrive. Sins 2 more than likely will be the Tec, and Advent combining their forces with what is left of the Vasari to fight this Unknown Enemy. According to Diplomacy lore the Vasari have nowhere left to go. The war with Tec crippled them. Some Advent sub factions are sympathising with both sides. Another expansion to cover the "ultimate evil" wont cut it. New ships, New tech trees. New features. New everything will be needed. Again this is pure speculation, but i dont see how they would pull off Sins 2 without the "Ultimate Evil".
Dev's said a long time ago that they would consider a campaign. If not for Sins, but for Sins 2.
If there is to be a Sins 2 we are talking about at least 2 years down the road minimum. (again pure speculation).