Taking to the forums, he confirmed that enemies will be stronger in New Game+, you can select any difficulty level, and the Death March Trophy/Achievement can still be obtained.
As for what carries over and the starting level of your character, he explained:
What carries over?
Player experience (if you’re below level 30 when finishing the vanilla game, you will be bumped to level 30 for the start of NG+), items (excluding: quest items, books & letters, Gwent cards, usable items, trophies), money and alchemy recipes.
What’s the starting level of the character when beginning NG+?
The same that you had when finishing the game, if above 30, otherwise you start at level 30. You also get a free Clearing Potion at the start so you can reset your skills and try a different build.
Momot added on Twitter, “Witcher gear does carry over.”
Meanwhile, an upcoming patch for The Witcher 3 will fix an issue where Vernon Roche doesn’t talk, while the Triss romance update won’t be included in the next patch.
While the New Game+ DLC is undated at the moment, we know it won’t arrive this week.