I think the biggest mistake for the franchise was their infatuation with HL2.
>two episodic games where the plot barely moves
>a prequel with a retcon for the second episode
>all predicated on hoping the player just LOVED HL2 and its characters
Imagine a world where Episode 1 didn't retcon the G-Man stopping time and taking you back into stasis for "the next adventure". The episodes don't happen. Half Life 3 could be anything, it could take place ten, twenty years in the future. What humanity does with the scraps of the Combine's transhumanist research, cults proclaiming that the future of human evolution is in their hands, nature slowly recovering after the Combine rape and pillage that took place over the last twenty years. And then, boom, another invasion, but this time, the Combine isn't bothering with augmented human soldiers and oversized biomechanical insects. Hell, take it out of Earth entirely, maybe Freeman's next assignment is liberating an alien city, except the people you're fighting turn out to be humans 200 years in the future. There's untapped potential in the concept of being a time-hopping agent for a space-warping entity.