I'm tempted to say that the more interesting exercise would be guessing how each of the usual suspects--myself included--might respond.
Well, there. I've went and said it.
The short answer would be "Almost everything." The longer answer is in fact too long, so let me just give a more refined answer, if nearly as short. I would push the timeline of human existence back. Knowing what we know now--that a civilization to rise up a trillion years from now will make all of the same scientific discoveries we have, formulate all the same hypotheses, do everything right, and still come to the false conclusion that the universe consists of our galaxy and nothing else--I can only wonder what truths we have lost forever in being alive during this particular age.