I haven't read the full thread so maybe these changes are already known, and I already posted these in a couple discord servers so I may as well post it here. I just tried premade co-op for the first time in the demo. Here are the changes from Nioh 1.
1. In Nioh 1, if the co-op gauge drained all the way, you failed the mission and had to restart. Here, it's the same UNLESS your party is at a boss. At a boss, even if people die so much the co-op gauge empties and you can't revive your party any more, as long as one person stays alive and manages to beat the boss, your party succeeds.
2. In Nioh 1, in premade co-op you couldn't use shrines. You could interact with them and get temporary buffs but you couldn't use the shrine menus. You can in Nioh 2. Everyone in the party can level mid mission, do offerings, switch spirits, etc etc.
3, Three player premade co-op is fucking hilariously fun. Nioh 1 only had 2 player premade.
NOTE: Premade co-op refers to co-op through "Expeditions" from the Torii Gate on the world map, not summoning a visitor mid-mission through an Ochoko cup.