Reality check:
- Source 2 is an extremly outdated engine that is 1-2 decades behind the competiton. Even HL Alyx still didnt have level streaming, like wtf! UE3 & others had that in 2004!
- In order to make a HL3 that meets the crazy high expectations of modern players, they would either have to use a modern engine, or make a new one.
Using a competitor´s engine is unlikely and making a new one on the same level as Anvil, UE5, Frostbite etc, requires a very large amount of engine coders.
On top of that, you need another large team to develop a Triple A title with the new engine.
- Valve does not currently have the staff numbers or expertise to pull off anything like that, and we havent seen them going on some sudden hiring spree of experienced devs either.
- Half Life is an IP only old people like us care about. Releasing some AAA HL3 to a very limited audience without having remade HL1 and HL2 first will lead to disaster.
Conclusion:
If we ever see a HL3 - which I doubt - it will most likely be done in an upgraded Source 2 variant aka Alyx and not try to compete with anything that is on the market.