I think that the Enderdragon shouldn't be the only boss in the game. I think there should be a few more, and they should reside in the overworld. So, my idea is that you can find these massive pits in the ground, and the pit would vary by biome. Only a few of these would spawn per world. These would be where you fight the boss. Now, you'd probably think that you could easily get out. Let me explain further. In the middle of the pit, there would be a small monument or something with a 2x2 pad of half slabs on it. To start the boss fight, you would step on this and the fight will initiate. But, you cannot escape Some easily identifiable ring would form around the pit on the edges. You cannot cross this until you win the fight. Therefore, you cannot leave until you win or die. Once you beat it, you can rechallenge the boss whenever. Here are some concept photos of a swamp arena:
I like the idea of overworld bosses, but not sure how they should be implemented. However, this gave me an idea for an awesome area in my LP world, if you don't mind me stealing it.
I was also thinking that it should be a separate boss dimension, which needs a portal to access. Im also working on another concept of an overworld boss arena as another dimension.
Sounds interesting, but I would rather a Nether boss or something, like in the Nether Fortresses. That would add a new use for the Nether, and it would be harder to get out.
Boss Pit:
The "initiation" pad: