HLX (Half Life 3) is releasing late next year and will have a digital Artbook and digital Soundtrack release. The artbook will have concepts from even before HLX and commentary from current and former Valve artists.
There's a new six-legged alien spiecies or enemy similar to the Antlion. They have spikes all over their body, two antenna, several manibles on their mouth and glass eyes. The name of the creature is related to the Spikes.
Combine themed Horses and Warbeasts which are ridden by enemies. There is a new Drone enemy inspired by the Manhacks but larger.
The Combine themselves look similar to Alyx although their designs are have been changed to seem more militaristic. Some Combine models have visible duct tape and scratched up gear on their persons.
Concept art for barrels blowing up, being destroyed, spilling liquid, and so on. Trucks with oil containers, large gas cylinders, fire extinguishers, that sort of thing.
There is an art piece of a destroyed space ship (parts blown away, floating in space, dead Combine bodies) in either space or some alien planet.
Weapons are modern inspired but are entirely fictional. Shotgun, Pistol, Crowbar, SMG, etc. This may just be because they're Combine weapons.
Gordon Freeman looks like Gordon. If you were expecting Valve to redesign him, they haven't (at least based on the art).
If you're expecting HLX to be announced this year, just don't...