I'm a DS and Bloodborne fan, and I also loved Nioh.
I'm about 20-25 hours in, defeated 2 bosses and a dozen or so mini-bosses.
This game is AWESOME!
The scenery is breathtaking.
The movement and traversal are exhilarating.
Combat is easy enough against normal enemies, but against mini-bosses especially you will need to bring your A-game. Where DS games allowed you to breathe between strikes, Sekiro needs you to persist in your attacks until you break the opponents posture, after which you get a deathblow, and your opponents will try to do the same, which makes combat very tense.
Minibosses also require 2 deathblows to kill, BUT you can stealth strike every miniboss to initiate combat, which counts as one deathblow. (I've only had two minibosses so far against which I couldnt initiate combat from stealth, and needed to deplete 2 healthbars or deal 2 deathblows.)
The different tools and upgrades for them also give you lots of variety.
It's seems to be also pretty big. I'm not sure how many bosses it has in total or how many zones, but I'm in awe of what I got so far.
My game of the year, unless TLoU2 manages to come out this year and then we'll see ;)