Being such a huge HL fan, I really like the idea of making content for it. I have in my drawer stuff for what was supposed to be the second chapter of Riot-Act, the add-on I collaborated on years back, and other stuff of mine I never developed. The HL2 world is so rich is not difficult to develop ideas for.
There is definitely an audience as well, there are a lot of mappers and community, and a great mod would surely ignite the interest... but taking over what Valve has been working for years? I don't mean to be negative, because I think we can come up with a great project, just worried it could be a big "waste"... in the sense that for as good as it can be, if Valve doesn't put a stamp on it, it can't be canon. And that would be quite sad. So I would rather work on a spin-off. As short and with as much "bang" you can put in it. Valve likes it? You might get access to Source 2 and work on an approved sequel.
Flying too high? 
Quote from Sprony6 hours ago, Sprony said: We are continuing to use Source 2 as our primary game development environment. Aside from moving Dota 2 to the engine recently, we are are using it as the foundation of some unannounced products. We would like to have everyone working on games here at Valve to eventually be using the same engine. We also intend to continue to make the Source 2 engine work available to the broad developer community as we go, and to make it available free of charge.
Yes that's basically what I said. We expect Source 2 to be released but won't be for a long time. "HL3" on the old engine is not as exciting.
OT: @FMPONE I meant to PM but didn't get around to it... in the past you shared a couple shots of HL2 environment, you ever worked on that SP map again?
