We say 'another' because STALKER 2 was actually announced back in 2010, when the studio confirmed it was building the game using a new engine. However, GSC shut down a year later and thus the project was cancelled, though the company was later resurrected in 2014.
There's not an awful lot to go on right now, although that probably isn't a surprise considering STALKER 2 is apparently targeting a 2021 launch window.
There's also murmurs of the game using the Unreal Engine 4 based on a Tweet by Epic Games' Sergey Galyonkin, though this hasn't been confirmed.
The original STALKER was followed by the prequel STALKER: Clear Sky and a direct follow-up, STALKER: Call of Pripyat.