There is a useful detail in Valhalla's new patch notes for anyone with an unfinished save. You need to complete Holy Day to unlock New Game+. You don't need to finish absolutely everything. Holy Day closes the Hamtunscire arc, and Ubisoft says it chose a point before the true ending because the game is so big.
The update arrived on November 5 for PC, PS5, PS4, Xbox Series X|S and Xbox One. It is free. New Game+ carries progression into another playthrough, with the difficulty rising on subsequent runs.
What happens to repeat rewards?
Equipment you already own and books of knowledge you have already found can now give you Mithril in New Game+. The material is used to improve divine gear, whose upgrade limit has gone from 13 to 27. Shops offer other ways to obtain Mithril if you would rather stay in the endgame.
That last option is worth keeping in mind. A New Game+ announcement can feel like an invitation to abandon whatever you were doing and begin again immediately. Here, there are additions to look at even if another trip through the story isn't what you want tonight.
For the inventory you left behind
Ravensthorpe now has a stash for equipment and runes. The Rune Forge has more than 300 engravable runes, plus a Superior tier that needs Mithril to reach. There is also an optional Extended Progression Mode that includes equipment upgrades, runes and masteries in your power score.
Our first stop on returning would be that stash. It's tempting to treat every old item as something you might need later, then never do anything with it. Put the spare pieces away, leave out the ones for a build you actually want to try, and the decision to start a new run gets a little easier to make.
None of that tells us whether another full playthrough will hold your attention. It does give a returning player more to consider than simply starting the same character over. Check the quest log before making that decision; you may already have the unlock you need.