IGN has an article with some more info.
Apparently this is an alternate universe and where XCOM didn't win. Not happy that it is not a straight sequel.
"Solomon painted an alternate version of how the war unfolded: “When the aliens showed up, XCOM suffered massive casualties, and governments around the world crumbled in face of popular support to surrender. Then, the Earth was quickly overrun. And so, 20 years into the future, the world is a very different place. The aliens rule Earth from giant shining megacities where all the people of Earth are flocking; that’s where they’re promised an easy life, a secure life free of disease.“"
Some details on the soldier classes.
"First of all, we have a new team of battle-hardened XCOM soldier classes that are similar to Enemy Unknown’s, but distinct. The Sharpshooter shares a lot of DNA with EU’s Sniper, but can also specialize in using a pistol as a primary weapon instead of a last resort. The Ranger is the evolution of the Assault class, still specializing in close combat with shotguns and the like, but now comes equipped with XCOM’s first lethal melee weapon: a machete-like blade. The Grenadier? He (or she) blows things up, much like the Heavy. The Specialist replaces Support, and uses a hovering drone called a Gremlin both in combat to stun enemies and buff allies, and out of combat to pull off tricks like long-range hacking. There’s also a fifth mystery class that Firaxis is saving for reveal closer to launch."
You can loot. Nice.
"Inventory is set to be much more important this time around, too. “Loot is a thing now,” said Solomon. “One thing for the soldiers is that they have to make do with the things they’re recovering from the battlefield.” This doesn’t extend to the kind of on-battlefield inventory management X-COM: UFO Defense fans remember, but rather a sort of “backpack” into which you can throw loot items recovered from fallen enemies or allies. Once you return them to base, you’ll be able to equip them for the next mission. What that loot is hasn’t been detailed yet, but we can see in the trailer that XCOM soldiers have in some cases literally duct-taped upgrades to their weapons.
Loot will serve an important function on the battlefield, too: in the same way that Enemy Within added the Meld resource to incentivize us to charge our soldiers into danger (instead of slowly advancing your troops at an ultra-conservative pace) by putting an expiration timer on valuable collectables, XCOM 2’s loot will have a countdown. If you don’t reach loot items within a few turns, they’ll explode into less-valuable weapon fragments, which is what happens to gear in Enemy Unknown if you don’t take the aliens alive. So there’ll be increased risk attached to the best rewards, because the Advent soldier or alien who just dropped something shiny might have still-living friends in the area.
If one of your troops is rendered unconscious, wounded, or killed in action, one of their teammates can sling the body over a shoulder and carry them out. When resources are scarce, it’ll be crucial to save their equipment even if you can’t save their lives."
Sectoids have been upgraded with human DNA.
"Case in point: the Sectoids have taken this opportunity to incorporate human DNA into their own, rapidly changing their species from four-foot-tall imps into imposing seven-foot-tall monsters with beady eyes and a brand-new set of unsettling pearly whites. Worse, they’ve enhanced their psionic abilities to include mind control from the get-go, making them extremely dangerous for any one soldier to take on by themselves. Despite their more human-like legs, Sectoids have apparently not yet evolved the ability to wear pants, but still have greyish-pink skin and an otherworldly glow emanating from their internal organs. Though we saw an XCOM Ranger slice through a Sectoid with a melee attack in the trailer, Solomon assured me that a move like that would only be possible in-game if the Sectoid had already been softened up."
The Snakeman is a Snakewoman, and they are called Vipers. Looks more like a cobra to me.
"In addition, the XCOM 2 trailer showed us the snake-like female Vipers (a clear reference to the Snake Men of X-COM: UFO Defense), who have the capability to not just immobilize and slowly crush a soldier with a constriction ability similar to Enemy Within’s Seekers, but can also yank them right out of cover with a frog-like tongue. Beyond that, background images hint at the return of the psionically powerful four-armed Ethereals, and we’re told many other XCOM enemies will be back along with a complement of all-new alien invaders."
No relation between Advent and Exalt.
"Obviously, this kind of creature would render the early game a miserable, punishing experience wherein the typical XCOM rookie would be ground to a pulp if there was more than one or two in play at once. Hence, we have a new form of fodder against whom our lowly recruits can prove themselves and level up: the Advent. So far we’ve seen two ranks of these (probably) human collaborators, whom Solomon clarified have no direct relation to the Exalt forces from Enemy Within. These rank-and-file troopers wear black armor, wield magnetic weaponry, and maintain order in the alien-controlled cities. We’ll have more on the aliens and the Advent later this month in our XCOM 2 Enemy Profiles."