I guessed they would do this ages ago, the obvious game choice would be Half Life 3, second? it's possible it could be Counterstrike afterward since CSGO isn't considered a follow up in a way.
I hope this means they are making the engine better with physics and dynamic environments, that would make a game like L4D3 be able to have much more random environment changes.
Soon as such a game is announced i'll be ready to upgrade. Hopefully they aren't waiting for next gen consoles to release before they bring it to PC, they could bring it to pc and just downscale it again for consoles when they come out anyway.