UPDATE 28/07/2015: CD Projekt has revealed how The Witcher 3's upcoming new game plus mode works.
Posting on the CD Projekt Red forum, community chief Marcin Momot said enemies will be stronger when you play the game again through new game plus.
Player experience, items and alchemy recipes all carry over. Of note: if you're below level 30 when finishing the vanilla game, you will be bumped to level 30 for the start of NG+.
You also get a free Clearing Potion at the start so you can reset your skills and try a different build.
Also of note: items that carry over exclude quest items, books and letters, Gwent cards, usable items and trophies.
When you start a new game plus you can select any difficulty level, and you can obtain the Death March achievement while playing NG+.
ORIGINAL STORY 27/07/2015: The Witcher 3: Wild Hunt's final DLC will be a New Game Plus mode.
This will be a free update, developer CD Projekt Red announced on Facebook.
While no release date is mentioned, the developer said "It won't happen this week though - we need a little bit more time to finish it."
The Witcher 3 has had numerous updates since its release in May. Some of these were pretty major, including one that added an entirely new movement system as many folks found Geralt's bulky lumbering awkward.
Two Witcher 3 expansions are planned, which should equal The Witcher 2 in length. The first of these add-ons, Heart of Stone, arrives October. Its follow-up, Blood & Wine, is due in Q1 2016.