from the looks of the last part of the intro, all 3 factions seems to be phase jumping unto another distant galaxy. could they be launching a counter attack to the threat or simply sending an expedition fleet to probe what or who the threat may or could be. this could be an opening introduction of a 4th race which they could be themselves warring with another race thus adding 5th or 6th race to the universe of sins. either way, this whole story line is like a big movie that makes me feel like i myself is a big part of.
simply speaking in a technical sense that sins of a solar empire is a "huge" single player campaign that uses all 3 faction leading to one big mystery, from the unity of tec, returning vengeful advent to vasari arrival. continuing in a united front to conquer and defeating the threat of their existence. i would call this one of the best epic single player game that's ever been released in history of our time.
edit: i had to change the word "ending" to "continuing". i would not want sins universe to end in any way shape or form at all!