Ok, yes I do know this would take alot of coding! But you Don't really die, you just wake up when you press the button at the top. This would appear after your last heart goes. Thanks for the baardie! And I like your idea! Boss fights, I always found this interesting. Just to start a mod now with bosses would take for ever. So if this mod were to be made by someone, then in a later release there would be bosses added.
You, my good sir/madam, have made me think of several bosses from games. Namely several SEGA bosses and all of the bosses from BHatGE and Sonic Colors for Wii (Fighting a giant ferris wheel? Hell yeah! Attacking a demented crow from taking over the world? Even better? Saving the galaxy while attacking a certain evil doctor while he uses energy from stolen aliens? Even better than Wolves [just noticed other mod reference lol]) Oh wait! I have an idea! This, if made, could be compatible with some of my mod ideas (I'm not a coder at all), like my BHatGE mod idea, my idea for a world generator (Islands! Woo!) and finally my somewhat improvisation on the SpeakerBlock mod. If you want to see the other mod idea please go here. I really like some of the ideas people have put out there-if only there were the modders to do it.
Anyway this seems like a nice idea-though if you play on peaceful difficulty ingame then it would be nicer to have a creative dream mode in which there are no mobs and you have all the game items at your disposal.