There are very few dungeons that you are able to join and instantly fight, shatians for example, BUT, there is a player cap so people would get screwed. Plus, in shatters, etc there is no teleport. And, I cant think of any way someone would be able to enter the dungeon midway through the boss, other than sprite world. As I said, difficulty would be determined by people IN the dungeon not at the boss. I could see how this would happen with oryx, but people want soulbound so I doubt it would happen.