Yeah sure, fighting the Enderdragon is challenging and all, but what if you want something else after you've killed him? I would like a mod that adds more bosses that could either spawn in structures throughout the overworld and the nether, or in the End after you've beaten the Enderdragon (turning the End into a boss challenge arena; every time you go back in, you get a random boss fight. If that happens, though, then the XP gained from the Enderdragon should be reduced). Some initial ideas:
Big Blaze - a giant-sized blaze that spins around and fires five fireballs at you, then must rest.
Big Skeleton - Fires multiple arrows per shot, or one oversized, one-hit wonder arrow, with a huge draw time: if you can hit him while he's drawing his bow, you can stop it. Perhaps generate a parkour room where you can climb up to hit him on the head for massive damage. If so, the only way to hit him on the head in this parkour room is to fall from the ceiling, then start all over. If you try to build your way up to the top of his head, then he can knock away towers of cobblestone and dirt.
Spider queen - capable of generating massive amounts of smaller spiders to protect her; huge weakspot of an eye
Kracko? - There's an enemy in the kirby games that resembles a cloud with an eyeball. He flies around, summoning lightning and occasionally flying down at you. Capable of changing the weather.
Chameleon - crawls along the wall, capable of changing his texture to match the block he's sitting on. In the chameleon's room (if that's how you're going to do it, nameless modder), the walls are made of a block that can switch between all 16 wools colors in MC, and there's a switch on the ceiling that can change the color of these blocks. Hit that switch, and it will take 3 seconds for the chameleon to match the new color, letting you aim at him with a bow.
Eyeball - if you've played Super Mario 64, you'll know what I'm talking about. A giant eyeball on the floor that shoots a beam attack at you, and who doesn't take any damage from any player attacks. When you run around him 3 times, though, he can't take it, and dissapears.
Ninja - a human-sized boss that runs much faster than you and jumps around the room, throwing shuriken at you and hitting you with a katana, occasionally teleporting in a puff of smoke. If you can hit him with a snowball or an egg, he will fall to the ground, stunned for a bit, letting you catch up to him and hit him.
Whale - a water-based boss. Because he's designed to move in water and you're not, getting in the water with him is a bad idea. Instead, stand on the water's edge and try to spear him when he gets close.
Creeper boss - a giant-sized creeper. He's weak to fire, though; hit him with a Flame bow and he's down in 5 hits; otherwise, it takes a lot longer. He can throw mini-creepers to hit you from a distance.
Robocreeper - he has an iron shield to protect him from your arrows (much like Wheatley); to hit him, you have to strafe to his left or right to get past the shield.
Big turtle - a miniboss with an impenetrable shell. To attack him, you have to flip him somehow; an explosion should probably do it. Be careful flipping him, though; when he rolls over, it might hurt you.
Phoenix - instead of diving towards you, like the Enderdragon does, the Phoenix prefers to attack its prey from afar - flinging both ghast-style and blaze-style fireballs at you. He even has a ray attack: the point of impact is show with a target on the ground or something, and you have to run away from the impact point to survive.
Ghost ship - an apparation with a mind of its own. Capable of aiming arrows + lit TNT at you. To attack it, you have to crawl up its sides, beat some ghost pirates, and attack one of 3-5 masts. Taking all of these down will destroy the ship.
There should also be some way to summon all of these mighty mobs for a rematch. Perhaps some sort of special Enderpearl? A book? A musical instrument? No clue. All I know is, I'm itching for a Boss Battle Minecraft experience.
And yeah, I know this is probably going to be a lot of work to whoever decides to make this idea a reality. Deal. Oh, and I'd prefer it if the mod was compatible with as much as possible. Like Forge and stuff, so I could play this using EE, or Industrial Craft, or Mine and Battle.
Some more ideas
Iron Colossus (big iron golem, climb up its body to attack his head)
Stone Colossus (less powerful Iron Colossus)
Yeah, probably. At the same time, I've always loved the Legend of Zelda boss fights, so I'd love to see something like that for Minecraft, where you have to be creative to beat the boss, instead of just wailing on it.