Personally, I don't really think story-based open world games really work with co-op, because a lot of what makes games like that work is just getting immersed in the world, and it's hard to do that if you've got other people around breaking the illusion. Further, I think having other people around encourages action, and sometimes you don't want to push forward in a game like Elder Scrolls. Sometimes you just want to wander around and see what you find. That's why they work so well as isolated single-player experiences.
I think co-op is best in games with a minimal focus on story, dialogue, and cut-scenes. Like, Vermintide 2 has a story, to be sure, but it's in the background for the most part. You see that with similar games like Borderlands and Left 4 Dead. They work because they are relatively constant action that keeps the players engaged and working together.
None of those games is really anything like Fallout or Elder Scrolls, though. Divinity is more about creating a character and role-playing as him or her, and it's not really an open world game either. I didn't play Neverwinter, but as I recall, wasn't that similar in that it wasn't very story focused like other D&D style games? The same is true of Borderlands. It has a story, but it's mostly just a loot collectathon like Diablo. The story is there, but outside of some funny dialogue it's not what I would call important or necessary. It's more of a basic idea to justify going around, killing an entire planet's worth of creatures, and collecting newer and better loot.
I think part of what makes Fallout and Elder Scrolls stand out is that sense of vastness and isolation. You're alone in a hostile world and your job is to explore it and unlock its secrets. I just don't see that working that well with co-op.
I don't consider Sea of Thieves to be a survival game. It's totally different. You don't build things. You don't need to do anything to stay alive. There's no crafting. When you die, you respawn thirty seconds later with all the stuff you had when you died. Sea of Thieves is basically just an open world multi-player shooter with some PvE elements.
