The Witcher 1 used RedEngine, which was the NWN Aurora Engine with a completely re-wriitten renderer. NWN2 used an evolved Aurora Engine, which Obsidian called the Electron Engine.
I would actually say the NWN1/2 engines are much more capable of enhancement than BG1/2, because they were specifically designed for creating additional content, modules, persistent worlds and new story modules.
Beamdog seem to have a roadmap to update NWN1 ( for which they have the rights ), such that old content continues to work, while enabling new content of much higher graphical quality. They have even published a document describing the data formats they will use and rendering techniques they wish to enable.
The old content also benefits wherever possible from updated rendering code; for example, a new lighting model that they have recently been previewing in news feeds ( not sure when the updated code will actually appear ).
There is also a DLC/game for NWN:EE written by Silverstring Media/Phantom Compass called "Dark Dreams of the Furiae", a Planescape story coinciding with the events of Descent into Avernus.
I haven't played it myself, so I can't vouch for it, but it shows there is still some life in the old NWN engines.
