If you were wondering whether The Witcher 3's New Game+ mode makes the game harder, I have some good news for you: It does!
New Game+ was just added to the game as the last of developer CD Projekt Red's free post-release downloadable content. It's a cool final addition, since many people (like me) have finished the game and want to try it again and make different decisions. And maybe see if they can keep from screwing it up with Triss and Yen this time.
I fired up a New Game+ and selected "Blood and Broken Bones" difficulty again, which is what I played the game on the first time. I played through the prologue, then found myself standing by the fire with Vesimir. I had pretty much all of my gear, usable items, and alchemy/crafting items.
You'll notice I also had a fresh set of that snazzy-looking Kaer Morhen armour, and, interestingly, its armour rating has been juiced up to 160. It's still not as good as my mastercrafted armour, but still a substantial improvement.
I've been playing for a bit now, and opening enemies have all been level 31. I'm level 34, so they haven't given me too much trouble, but I can see how things will get much harder as I go, particularly because I already have the best armour in the game and probably won't change it. I've been getting a ton of XP for everything, too -- I got 630 XP just for beating up the guys in front of the bar in White Orchard.
I'm assuming that difficulty increase will hold throughout. I'm still playing through the opening act in White Orchard.