Santa Monica Studio decided to be honest very early on and announced God of War: Ragnarök would launch without a New Game+ option, but that this would come later on. Fortunately, they've followed the same recipe as God of War.
Because you can now download update 04.00 for God of War: Ragnarök, and there are quite a few other goodies besides the implementation of New Game+. These other features and changes include:
4 new armours (including the long-requested bear cloak Kratos wears in the game's intro)
New colour variations for 13 different armours
A new level cap simply called 9+ that also makes it possible to improve your skill mods and stats even further
A new Spartan Aspis shield with a tighter parry window that deals more damage when done succesfully
New enchantments
More options in Niflheim's training area
Tweaks to some endgame bosses when playing on New Game+
Black and white render mode on New Game+
The option to skip cinematics in New Game+
A bunch of other things you can read more detailed information about here.