At this point, they need to save "innovation" for other games and just make a really tight FPS game that wraps up the story after Ep2's cliffhanger. Seriously.
Make it to the usual high Valve quality standards and release it, and we're off your backs, Valve. Just do it. Make your dreams come true (etcetcetc)
They really don't. Completing the story arc of a cliff hanger is low hanging fruit, and an underwhelming return for HL as a series. As an episode or expansion perhaps, but it would still be underwhelming. It wouldn't live up to any expectations for what a new HL game has brought with it in the past. I am more than happy for them to take as long as it takes to bring out something that is remotely close to the leaps HL games have brought in the past. It could take another 5 or 10 years; it doesn't make a difference. The game has to at least attempt to live up to expectations, and that means innovating in a meaningful way that isn't something as forced as just another sequel to complete a story arc, better graphics and an open world or other regular tropes