In an interview with Polygon, lead designer Ananda Gupta talks about the expansion is a logical progression from the original game. In Enemy Unknown you progressed by gradually incorporating alien tech into your own set-up.
“How could we take that even further?” says Gupta? And the answer was [that] we want to let players incorporate the alien technology into the very bodies of their soldiers. You know: enemy within.”
Enemy Within introduces the MEC suit, the Mechanized Exoskeletal Cybersuit. The new Mech Trooper class wears the suit and can be equipped with heavy weapons such as a grenade launcher or flamethrower.
Enemy Within also introduces a new resource known as ‘meld,’ which is what allows you to mix human and alien biology together. It’s a resource that can only be captured in battle.
Other new features include improved inventory management, offline squad management in multiplayer, and a bevy of new maps. The trailer is below.