There's a lot of things I don't get about this debate that I see throughout the thread.
1. "Expectations will be tremendous for Valve after all this time"
That's not how it works, or how it should work at least. It's one thing for a project to be on track for many years and get tons of effort put into it- it's another for an obviously dead project to get brought to life at some point. For a recent example, the hype/expectation cycle for The Last Guardian built from the point SotC finished to its reveal to probably a year or two after that before it began to plateau as the project floundered. By the time the game actually came out, I'd say my expectations were lower, if anything, than if it had come out on time.
2. "The game needs to be revolutionary"
No, it doesn't. The best HL game is Episode 2- a title that, largely, brought nothing new to the franchise, much less the genre. These games succeed for their strength in execution, not the novelty of their concepts.
3. "The franchise has been left in the past"
The industry is not putting out games with the quality and nature of Episode 2 with such frequency that an Episode 3 or HL3 would not be welcome. I'd also add that Episode 2's campaign remained unmatched in single player FPSes till potentially Titanfall 2 last year, and even then that's a toss up. Beyond my feelings on the quality of the franchise, there aren't even other series that do what HL does- a linear story driven fps blended with puzzle solving and the occasional vehicular segment. It would actually be fresh in the current environment simply because no one else even goes for that specific mix.
As for the OP's question itself, I do agree with others sadly that- given the strength of HL relies on its execution- Valve really is- or was- what made it special. I just don't see another team nailing it. (Although, on the TF2 note, if Respawn- with their Source experience and amazing TF2 campaign- theoretically had the IP and the old HL writers/Valve agreed to hand them their vision/script, I'd definitely give it a shot)