Supposedly it's twice as big as Enemy Within.
The first part is basically Alien Hunters II. You get three "Chosen" - humanoid (ADVENT-aligned I'm 99% sure) aliens that will be reoccurring foes. They also get more powerful throughout the course of the campaign. They're apparently meant to basically be the arch nemeses of XCOM.
Then you have three rebel factions that you can recruit from, but apparently they're hostile to each other.
The first faction is alien-human hybrids that defected from ADVENT and have some pretty cool armor to boot.
Then there's a super-psionic faction. Their units use a resource called "Focus" that accumulates over a battle. You can spend the Focus on powerful abilities or keep it to make the unit generally more powerful.
I missed the third one.
EDIT: Oh yeah, the zombies.
They're a new faction that can randomly appear in missions, and they attack both sides. I believe they're called "The Lost" and are supposed to be located in the ruins of the cities that were subject to heavy fighting during the aliens' invasion. Basically, they can pop up at any time, are attracted to gunfire/sounds, and will attack anything that moves, including ADVENT.
Tons of other stuff as well. New classes, new map types, new items (including Halo-style energy swords!), new strategy layer stuff, and much more.
XCOM 2: War of the Chosen Announce Trailer
EDIT: Even more stuff! This comes from
the updated official website.NEW FACTIONS & HERO CLASSESThree additional factions have emerged to strengthen Earth's resistance – the Reapers, Skirmishers and Templars – each with its own unique abilities and contrasting philosophies. These factions provide powerful hero class soldiers to aid in missions and new opportunities for the strategy layer.
THE CHOSENThe Chosen are the most cunning enemies XCOM has ever faced, each with unique strengths and weaknesses that will be introduced with each new campaign. The Chosen are on the hunt for the Commander and will kidnap, interrogate, and kill XCOM’s soldiers to complete their objective. The Chosen can also invade the strategy layer and ravage XCOM’s global operations. Find and raid the Chosen’s strongholds to defeat the enemy for good.
NEW ALIEN & ADVENT THREATSA deadly new alien known as the Spectre, capable of creating dark copies of XCOM soldiers, has snuck onto the battlefield. Adopt new tactics to counter it as well as the explosive attacks of the ADVENT Purifier and the psionically charged ADVENT Priest.
NEW ENVIRONMENTS AND MISSION OBJECTIVESEngage in tactical missions across new environments from abandoned cities devastated by alien bioweapons during the original invasion, to underground tunnels and xenoformed wilderness regions.
ENHANCED STRATEGY LAYERManage XCOM’s relations with factions and counter The Chosen’s operations from the Avenger. Employ new Resistance Orders to prioritize your personal strategy. Soldiers, scientists, and engineers can now be deployed for Covert Actions that award supplies and boost faction favor if successfully completed.
GREATER CUSTOMIZATION & REPLAYABILITYSoldiers can develop bonds with compatible teammates for new abilities and perks. The SITREP system dynamically adds new modifiers to the tactical layer to make sure every mission provides a unique challenge. Advanced campaign options allow for finer adjustments to game length and difficulty.
CHALLENGE MODEUtilize the perfect strategy in new regular one-shot community challenges to claim the top spot on the global leaderboard.
SHARE THE RESISTANCECustomize and pose your soldiers, then add filters, text and backgrounds to generate your own unique resistance posters that appear in-game and can be shared with friends.
This looks amazing.