Don't think they ever should made HL2 Half-Life 2. I liked the whole plot with Combine and Earth being taken over, but It should have just been a test with source. If they took out headcrabs and Zombies, Just left the Synth (I count the Striders, Gunships, Carriers and Ant Lions part of the synth "Species", not just the things you see on the conveyer belts), and Combien forces(the different soldiers, police, and stalkers) in, would have been a much better game, there was no need to be connected with Half Life, Although a good sequal to HL is a good idea, Continuing the storyline, and ANSWERING question like who the f*** is GMan would be cool. Ohh, and in HL, the headcrabs were far overused than people stated in this thread, As soon as I went into a vent it became habit to be prepared for a "Surprise" attack from a head crab from a T joint, or a corner, they wern't used well in the game, at least zombies were a little creative, Pulling people through vents, smashing doors, and in Hl2, they had some cool traps.
Sorry got of track from original post, but the other headcrabs were going to be in the game (I have the collectors edition, it came with a little book with some concept art, and some bio) but they didn't do it. So instead of yelling at them for putting them in, yell at them for not putting them in HL.