Well, this is all down to speculation right now. There certainly seems to be something brewing though. It almost feels strange to imagine a sequel to CSGO right now, or even just a new title in the series, since CSGO is still going strong. The reason that this has been brought up again is that two new executables have been uncovered by an NVIDIA Driver update:
csgos2.exe
cs2.exe
These are listed under the Game Profile 'Counter-strike 2'. It definitely sounds like a real development - though Counter-Strike developers Valve tend not to give much away themselves.