Alongside a 14th December release date, CD Projekt Red did actually give us a brief insight into what The Witcher 3: Wild Hunt - Complete Edition will include. According to the game's official press summary, the enhanced PS5 re-release will sport all of the bells and whistles that you'd expect, while also throwing in some potential surprises.
The PS5 edition will feature "dozens of visual, performance, and technical enhancements over the original". This includes things like "ray tracing support" and "faster load times" — but this is where things get interesting.
There's mention of "a variety of mods integrated into the experience". Now, The Witcher 3 has always had a healthy modding community on PC, and people have added all kinds of stuff to the game over the years. It sounds like some mods may have been cherrypicked and officially added to the adventure. However, there's no indication of how extensive this mod integration will end up being, so we'd recommend tempering your expectations — at least for now.
And lastly, the press release reiterates that Complete Edition will come with "new additional content inspired by The Witcher series from Netflix". We imagine that this includes things like new armour sets, or maybe even fresh cosmetics for Geralt.