I want a sort of sneaky, "wall shadow" boss.
A sort of wolf perhaps, or humanoid. Not that large or powerful, but fast, and the ability to go through walls. The arena is a maze,the creature can strike from any direction, and it's just plain scary.
The main flaw is it doesn't give much incentive to go anywhere in the maze, perhaps a dual boss, with the second one being a large, lumbering minotaur-esque beast, similar to the Taurus demon but larger, probably taking up all of the corridor. You REALLY don't want to get in front of him, you have to take advantage of the maze to get behind, take some swimgs, then start running (he has a pretty slow turning speed, so it's quite viable)
So you got a sneaky bugger striking at you from the shadows, and you don't want to stay too long in one place eyeing each wall for when it strikes, or the minotaur will find you and charge you.
Needs a little more thought and work, but I think the general idea could be cool.