It's essentially a system of new resources: political capital, fear, and respect. The diplomacy system is certainly the most interesting element for me, mechanically. Here's the trailer, which dashes through a lot of the new delicious alien content: Both videos and all the details are below.
That's what has been detailed in not only a new trailer but a full, hour-long livestream that took place yesterday, heralding a free weekend for the base game. They also spoke about the new diplomacy system, in which you interact with the leaders of opposing factions in hopefully more realistic ways. In our initial preview of Beyond Earth's Rising Tide expansion, Firaxis were keen to make it clear that the expansion would have a heavier sci-fi focus than the base game.