After you beat the game(if you ever do) I think there should be a quest to return to every world you've unlocked and battle a new boss from that world. You guys can read through or post your own on what the boss of the world is. Note: these bosses are the complete overlord's of the world so they are not too easy.
World:Wizard city Location: firecat alley Boss: inferno beastmaster Spells: all fire and astral spells ever Life: 44,000 Cheats: whenever you heal he says "Healing is for chumps" and takes away the life you gained. Also every five rounds he casts a fire elemental minion for free.
World:Krokotopia Location: hall of kings Boss: threba the conquerer(thrae-buh) Spells: all balance, astral, and myth spells ever Life: 50,000 Cheats: can use power pips in school of ice. After she is at 5,000 or less she casts bartleby for free
World: Grizzelhiem Location: ravenscar Boss: zalath the murder Spells: All death and balance spells ever Life: 2,000 Cheats: after you kill him he unleashes his true form which is same with 47,000 health. Can us power pips in balance.
World: Marleybone Location: ironworks Boss: Alyrer the creator Spells: All storm and astral spells ever Life: 10,000 Cheats: Every time he is dealt damage he summons a kraken minion with equal power to the one in nadvalier. Everytime a kraken dies he gains 800 health