A subtle "Special Edition" or "vanilla plus" version that adds bug fixes, replay value, extra functionality, content and atmosphere to the game, all while retaining that overall vanilla feeling, rather than the feeling of a vanilla game with various mini-mods tacked on as an afterthought.
In v0.9 of Call of Pripyat: Special Edition, there will be a lot of changes to NPC graphics, sound, and gameplay elements that really help improve the atmosphere of the Zone. Not all of these changes are large, but they do add up.
For example, the number of Duty SOS messages you receive in Jupiter have been increased, randomised, and now include lots of static and other effects to add to the atmosphere.