The new game is set twenty years after the events of the previous ones and features a bit of an about-turn from the end of those games: the aliens have invaded, again, but this time in much greater force and have won, occupying the planet. XCOM has been forced underground, waiting until some degree of complacency has set in on the part of the aliens, and then striking back unexpectedly. The game features a mobile base, the Avenger, and XCOM teams launching attacks on alien forces both in rural areas and inside the gleaming, futuristic cities they have built for their human sympathisers/drones. There is more of an emphasis on stealth and guerrilla warfare this time around, with the game apparently drawing inspiration from the third of the original 1990s XCOM games, X-COM: Apocalypse.
The game will also use procedural generation to create battle maps, rather than simply rotating through the same eighty-odd default maps as in the original game. There will be new weapons, alien types and new character classes.
XCOM 2 is, unexpectedly, a PC exclusive with no console release mooted at the current time. Enemy Unknown did surprisingly well on console, so the decision to limit the release to PC only is interesting. Firaxis should be revealing why shortly, but there are theories that the new game will feature a much heavier modding focus than the previous ones.