I don't see how it's possible to create new bosses with MASCOT. It's just a level editor. It might be possible to create new boss maps (although the AI may not work on a different map), but in order to create new bosses, one would have to much with the code itself, and as long as Ben and Ambrosia aren't going to release the source code, the chances of getting new bosses seem extremely small.
Anyways, as for the poll, I'd have to say Goblin Chief. That fight was actually challenging, and required the player to do a lot of things at the same time. Xichra was perhaps a bit too difficult.
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"I don't say yes or no, more like the other way around"
Ankh Starrunner, The Diamond Blaster