Source: CD Projekt Red
The next Witcher game is coming, but we can't say when it'll arrive. What we know now is The Witcher: A New Saga Begins will mark the game's shift from CD Projekt Red's REDengine to Epic Games' Unreal 5. CDPR says this game marks the "beginning of a multi-year strategic partnership with Epic Games." And that it "covers not only licensing but technical development of Unreal Engine 5." It will help with tailoring the engine for "open-world experiences."
As Kotaku pointed out, the last full Witcher game, The Witcher 3: Wild Hunt, came out seven years ago. But it got regular updates, so the game was kept fresh. We aren't sure when the new saga will begin, and CDPR said it had "no further details regarding the game—such as a development time frame or release date."