On the brain, you'll be able to choose from Neural Feedback, which inflicts damage on any enemy attempting psi attacks (though doesn't reduce the chance of success) or Neural Dampening, which renders a soldier immune to panic and, should an enemy succeed in mind-controlling him or her, will simply knock them unconscious instead of turning him. Each will have two options to choose from (which can be swapped if you're willing to pay) and we've gotten the details on two of those sets.
Every trooper have five slots for upgrades: brain, eyes, skin, chest, and legs. Mechs sound impressive, but there's a reason to keep your best and brightest out of a set of metal pants: upgrading a soldier to Mech status disqualifies him or her from receiving genetic enhancements. Killing that guy just disables the shield. If another sectoid mind-merges with him, he gains a damage-absorbing shield, but he doesn't gain the vulnerability of dying if the caster is taken out. These armored sectoids remind me a lot of the mechs from the TV show Falling Skies, and have the ability to fire twice in a turn (much like the Heavy's Bulletstorm ability). Presumably, these upgrades will come from researching the one new enemy (of several promised) that Firaxis is prepared to reveal: the Mechtoid. Mechs won't be able to equip standard guns and items, but they'll have their own weapon upgrade tree that goes from mingun to rail gun to an energy weapon, and they'll be the first XCOM units to feature a flamethrower. They'll also have an ability that allows them to serve as mobile cover for XCOM troops, making it easier to cross no-man's-land stretches without being gunned down, a jumpjet system, and Damage Control, which reduces the damage taken for one turn. There will also be an upgrade that heals every XCOM soldier in a radius. We don't yet know the full extent of the Mech Trooper's skill tree, but we do know that the first skill they unlock will be Collateral Damage, whcih will allow us to more easily target terrain (which Gupta says is designed to get us closer to the free-fire weapons of X-COM: UFO Defense). He or she will lose all of their current class abilities, but gets to choose from a whole new ability tree up to their existing rank. But this isn't a class you're born into like the existing Assault, Support, Heavy, and Sniper - this is one that, after the proper facility is built, you can move any soldier into. YES NOWe're also getting a fifth class of soldier: the Mech trooper.