After the announcement of the New Game+ mode in The Witcher 3: Wild Hunt - the last free DLC offered by CD Projekt RED for the game - the studio offered some more details regarding this new mode through the community lead Marcin Momot.
According to what he said on the official forums, level 30-players or higher will be able to keep this level when they start a new game in this mode. Those, however, have completed the game with a lower level, they will start a new game automatically at level 30. In addition to this, players will receive a Clearing Potion, which it allows players to reset their skills and try a different build.
Then Momot reassured players saying that player experience, money and alchemy recipes will carry over when starting a New Game+ (except: Gwent cards, quest items, books, letters, usable items and trophies).
At the moment, CD Projekt hasn't announced a release date for this mode, but they'll need "more time to finish it."