I'm not sure where the line is.. Like a super elite mob would be a fun/different challenge, but would it really be difficult in an open world environment? Second coil is challenging not just because of the monsters but because of the mechanics. All of the bosses would be a cake walk without the mechanics involved. It works well in coil because you are in a space designed to contain the party and mechanics, in an open world it would be difficult to duplicate (I think, anyways).

I really hope they do add something like this, that would require a full party. But it needs to be more than just a tank and spank, so maybe instanced content is the only way?