I've been asking for an expansion for a long time in which the two AI players turned on each other, or that there was another major faction in the galaxy which had no affiliation with you, but which the AI considered much more of a threat, at least initially. You as the player would have some power to influence or control these rivalries, to work out in your favor. The only drawback being that in the end you would still have to kill everybody to win, and that one wrong provocation could afford you an even deadlier enemy than the AI. I suppose this is the kind of idea that could make for a great mod.