After the tunnelling entanglement device is destroyed, the Combine can't get to Earth, and are handicapped. Earth is left to the humans, the Xen wildlife, and the Combine soldiers who might join the human cause. This three-way battle might span several episodes, and (if there is a non-episodic release) HL3 will take a major plot twist, as well as setting change/transformation. I think Valve has it right (if they do it that way) episodic releases to fill in the gaps, and larger installments for major plot twists.