I suggest having either 3 T3 bosses, or 3 T1 bosses.
Personally I think it makes more sense to have more T1 bosses, since there should only be one way to the Core, right? And there are already the Shadow Lair bosses, who are pretty much T3.
So it would be like,

Tier1______________Snarbolax.....(Boss)......(Boss)
_________________________\..........|.........../
Tier2________________Roar Twins....|......Jelly King
___________________________\.......|.. ..../
Tier3________________________Vanaduke

Anyone who agrees, please make a bunch of new topics in the forums about this - the developers only read the titles of topics!!!

-Colonimbus