Oh, I think that is a cool idea.
I remember back in the day when we were playing Everquest, which had open world raids, I had longed for some of the fights to be "walled off". Then instancing came around and I had thought [this was the early 00s] that instancing could be used to isolate boss fights. Instead instancing went in the direction of having the whole zone isolated.
Though I always liked my original idea of having a raid zone in the open world that allowed a raid force to engage the boss discretely.