Personally I have a few ideas on this. One boss that I think would be awesome to see would be potentially some kind of High King for the Fallen. He's not necessarily the biggest but he has the greatest experience. He's fast, hits hard, and is very hard to kill. Just an idea.
Some time ago there was a post concerning dragons. I opted for the idea of a Wurm (Much to the typically pictured MTG sense). Massive, moving among a large area, where you have to attack heavily armored parts of the wurm, until finally you force it to actually come to face, then you have a huge battle on your hands. Kinda similar to Terramorphous the Invincible in Borderlands.. but you know.. a good fight sequence.
I'd like to see a giant black metal ball that just chases you through a massive maze trying to crush you underneath it.
The maze is as much a boss as the boulder trying to end you.