While there are no official details, GSC Game World owner Sergiy Grygorovych revealed on facebook that they are working on a sequel to 2007's survival/horror game, tentatively scheduled for 2021.
There is an official website, but it just has a placeholder at this time.
This is not the first time S.T.A.L.K.E.R. 2 has been "revealed" since back in 2010 GSC Game World also announced the official sequel, but due to investors issues (related to THQ's demise or not) it was canceled in 2012.
In late 2012 German developer/publisher BitComposer claimed to have purchased the rights to S.T.A.L.K.E.R, which was disputed by GSC Game World as being incorrect.
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