As expected, Firaxis announced a full-fat XCOM 2 [official site] expansion at the E3 2017 PC Gaming Show, promising "new environments, new enemies and new XCOM forces". It's called War of the Chosen and the trailer shows those Chosen, the "ultimate enemies of XCOM". They are three champions - nemeses perhaps - and they will grow up in strength through the game, learning new skills and abilities.
On top of that, there are resistance groups to win over, new zombie-like hordes that will attack aliens and humans alike, and a whole lot more. It's "twice the size" of the original XCOM's expansion.
The three Chosen are the Hunter, the Assassin and the Warlock, and they're mirrored by the three resistance groups: Reapers, Skirmishers and Templars. If you're able to earn the trust and support of the resistance groups, presumably by assisting them and striking back against the aliens, they'll provide you with the most powerful soldiers ever seen in an XCOM game.
Skirmishers are alien defectors, Reapers are saboteurs and Templars have powerful psionic abilities. These latter sound interesting, gaining more power over time. You can save this to unleash their most deadly attacks, but you'll need to provide protection, or you can fire off weaker abilities more frequently.
All of this sounds like tactical combat changes rather than strategic level additions, but the Resistance and the Chosen will actually operate on the strategic map.
One of the biggest additions will be in the new environments though. Ever wondered what the abandoned cities of the XCOM 2 world look like? Well, they're not abandoned at all. They're full of people but those people have turned into monsters. Zombie-like mutants who attack in heaving great hordes. They're attracted by the sound of battle, and they're just as likely to tear through the aliens as your own forces, which will lead to three-way scraps in the city environments.
If the Chosen resemble the enemies produced using Mordor's Nemesis System even a little I'll be delighted. And I have to say, I was expecting Terror From the Deep as an extra underwater map layer, but this seems like it'll add far more than a new set of map terrain and enemy types.
There will also be some RPG type stuff, with soldiers forming bonds that'll make it even more heart-breaking when they die. So, new ways to die, new enemies to kill you, and more reasons to care about all the lives lost. Delicious.