New difficulty mode:
The way of the Nioh - level limits of player, guardian spirit etc. raised
Abyss mode available after beating the main story. What might be a new level of item rarity, 常世の品(literally "items of eternity") their ability is unknown until you leave.
The farther you progress into the abyss, the higher the drop rate becomes for them.
There will be a way to change items into 常世の品 and to raise their levels.
2 visitors can only be summoned in abyss mode.
new spirits, satyrid(type of butterfly/moth), and
nine tail fox
new weapon(s), tool(s) - no details were written
new transformations -
Maria and Hideyori Toyotomi
New trophies and titles
New armor with set bonuses
EDIT: some more info on the new item type, 常世の品
After some research, I found that they localized 常世 as "Yokai realm", so the new items will have a demon/hell localization most likely, contrasting with the heavenly divines and ethereals. It makes sense when you consider you can only get them from "The Abyss", which is more or less Hell.