My favourite of this weighty pack of additions to the already profoundly wonderful GalCiv 2 is also the rarest: Mega Events. At any point in a game of Dark Avatar, something absolutely momentous can happen. In my last game, a technology was suddenly discovered that could instantly terraform utterly uninhabitable worlds (still most of them) into the most verdant and luscious planets in the galaxy. On another occasion, a virulent nano-virus spread across every single planet, and I was first to research the cure. Selling it to the other civilisations made me so much money - and cost them so dearly - that I was able to crush them all in short order after that.
The effects might be wildly unpredictable, but despite that the events themselves are apparently not picked at random. They've actually cooked up an algorithm that analyses everything going on in the galaxy and triggers the one thing that would cause utter pandemonium in it.
This is just this kind of intelligent silliness that's always made GalCiv a joy to play, and worthy of even the most discriminating Space Miner's downtime.