Well, I note a patterN:
GC -- the basic game, with vague references to the Precursor history. Running through the expansions explored the mystery of why the Terrans and Altarans look alike, which suggested a deeper Precursor design.
GC2DL -- the game framework solidified and expanded. Much in-depth history concerning "where did the Precursors go?" The answer to which ultimately is, "The Dread Lords are still here. But to balance them there is just _one_ Arnor."
GC3 -- which I expect will be something along the lines of "The Return of the Arnor." Can't leave the galaxy to the whims of the Dread Lords; that would be a definite ultimate win by Evil. The story will have to have a serious, conclusive battle between the Arnor (returned from a galaxy far, far away) and the Dread Lords in their pocket galaxy. Such a conflict MUST be a galxy-shaking event. And given what effects that must be manifested during such a battle royale, the game's framework will have to be tweaked to accommodate those effects. And in order to have any kind of status quo in the universe, that battle _must_ end with the Arnor and the Dread Lords cancelling each other out, eliminating each other, and leaving the lesser races to sort things out for themselves.
Of course, there _will_ be a possibility that perhaps not ALL of the Arnor and DLs were eliminated. Gotta leave _something_ for a sequel, after all.
If you can figure what kind of plot elements will go into the Arnor versus Dread Lords battle (which will heavily employ their lesser races pawns), you should be able to get a pretty good idea of what new mechanics and plot elements will appear in GC3.