No proper stats yet, but..here are some ideas.
The Iron-crowned Tyrant
A weakened rip-off of Morgoth, this was some sort of god or divine spirit that was cast down from Heaven, although it still retains fearsome power.
Type would me major demon, he would resist blight(say 50%, or even 100%), the elements(say, 25%), and be VERY powerful in melee(maybe he should have Death Dance too, just because it's fun to have him attack you, your summons/allies AND other monsters

).
Should use a greater ego two-handed weapon(not necessarily a hammer).
The Mad Alchemist
Needs a better name(maybe Accursed Alchemist? Alliteration rules). A kind of evil Doctor Frankenstein, he started as a 'regular' Alchemist but turned to necromancy/blood magic. Throws special alchemist bombs which cause blight damage, and could perhaps be escorted by 'The Creature', another boss(a golem-like abomination with undead aspects, very powerful in melee - although it should perhaps be possible to find them separately...)
The Trickster
Needs a better name, again. Kinda gimmicky - basically a teleporting/stealthy boss that would debuff and use ranged attacks on you, and flee instead of fighting if wounded enough. It might also flee if YOU are wounded enough - it wants to play, not outright kill you(but if other monsters are around...)
Yes, it's MEANT to be annoying. If it stays out of LOS for a whole, it should disappear to fight another day. Should drop some powerful jewelry/magical item as a reward for actually managing to kill him.
The Mother of Monsters
Powerful nature type boss with summons/escort(snakes, worms..) and poison based attacks, also fast regeneration. As a downside, it could have a low movement speed. Inspired by [url=http://en.wikipedia.org%20wiki/Echidna_%28mythology%29]Echidna[/url].
Maybe killing her somehow affects other kinds of monsters(all appropriate monsters on the level rush to you to avenge their 'mother', or they become weaker or something else, to reuse my 'monster spawning affected by events' idea).