I'd still call AC6's system NG+, yes it features new content and a new ending options, but I feel like 80% of the game is still the same. Nier is different because the second and third playthroughs are very different with new perspective, new character controls and even if it still follows the same path up to a point, it's still altered enough to feel different. I think of them more as new campaigns.

I feel that NG+ in the most simple of definitions, is when a game allows you to replay the game with altered content from the original playthrough, but is still at least 75% the same game.