Coming fresh off Bloodborne into this and I'm wanting gaf to spoil some of the easily missable stuff in DS2 for me because that shit annoys me. Stuff like missable quests/areas/npcs.
You're better off consulting a wiki, because essentially everything in this game relating to quests and NPCs are missable. As such trying to make a list per area is basically impossible, and a lot of the time there's stuff you'll miss by just doing something in the wrong order that's basically impossible to not do unless you've got a guide sitting next to you or have played the game five or six times or so and know exactly where to go and when to get things lined up right.
I'm not trying to be disheartening, but the game is really massive and has tons of branching paths that you can mostly do in whatever order you want (especially in Scholar now that they changed how they handle Fragrant Branch of Yore), and unless you have more specific questions about an area/quest/encounter and where you've been in relation to that area before entering it, it's really hard to put you on the right path.