I’m glad I saw this, I actually thought this through long ago.
Before the trailer is revealed, Valve will hint at or openly mention Left 4 Dead at a gaming convention, then leave the stage briefly while a trailer appears on screen.
The trailer is ambiguously titled, only listing a “3” and nothing more. The narration opens up "It was so many years ago, they took our planet from us, they took our homes, our families, our lives… But I will never become one of them." During this narration, the camera will pan through scenes of destroyed cities, wrecked military vehicles, abandoned countrysides, and the like. After the narration ends, we are shown a small dilapidated cabin room in which 4 people stand around a table full of weapons, ammo, and supplies. They’re speaking about their plans, to escape or attack, when the leader shushes them. “Hunter” he says softly, everyone looks uneasy, one person reaches for a gun but he brushes their hand away “No noise”. We hear some growling and shuffling outside, and the group look even more concerned. “Go away…” one grits under their breath. Everyone jumps as the metal door starts banging “Oh no…” one mutters, then the banging stops, and a piercing shriek is heard from outside.
Then BOOM, a Combine Hunter bursts through the door as a crushed fast zombie is dropped to the floor in front of it. Cries of “IT FOUND US” and "HUNTER!"ring out as the 4 scramble for their weapons open fire. The Hunter immediately impales the closest survivor, then sprays down another with its flechettes before charging the third and smashing him into the wall. The last man flees through a side door and out through an old horse stable, but before he can escape the Hunter drops down infront of the exit from the roof. The man slips and falls into his back, screaming as he turns to crawl away, but it’s too late, the hunter drags him off camera and “HλLF-LIFE 3” flashes over the screen.
The End.