I think the "Enemy Within" deal is going to be about the development of technology running rampant due to humanity's exposure to the invaders and the willingness behind that to do whatever might be felt as necessary, losing whatever moral standards humanity had in the process. That was the entire thing in Enemy Unknown, with the scientist and engineer continuously commenting on how scary the tech was becoming, hoping it was worth it and that it wouldn't bite them in the ass later and if only it could have been developed for peace.
I haven't gotten all that far with Enemy Unknown yet myself, but all along I have felt that the "evils of technology" theme was pretty latent, as if it might be building up to something. I had been hoping (and now, I think it's fair to say that hope will be disappointed without the need for spoilers) that this would be Enemy Unknown's endgame twist- humanity actually overcoming their invaders by virtue of effectively employing their own technology against them factoring some of that uncanny homosapien innovation, at last utterly beating them back (we do, after all, as defenders have the luxury of our manufacturing centers and research labs right on-hand)- only to find itself grappling with a monumental rebuilding effort, shattered world economy, the need for defense measures against future invasions, and some all too-tempting means for some very Orwellian propositions at world leaders' fingertips'.
It'd also be neat to see a sort of "bad ending canon" spinoff where we're the invaders. Draw from a base pool of investment to determine the most alluring interstellar invasion prospects, then mobilize a fleet of Murican war machines screaming electric death and GMO soldiers accompanied by the third-world citizen human equivalents to sectoids, that we may seize that distant prize for the glory of the
Father World Wall Street.