I want to see the brothers fight Sekhmet, the Egyptian war goddess. In Egyptian mythology the Sun God Ra sent her to kill the Egyptians that stopped worshiping the gods, but she got too crazy and started killing indiscriminately. Fearing she would wipe out all of humanity, Ra filled a lake full of red wine and tinted it with iron to make it taste like blood. Sekhmet drank the lake dry, got drunk and passed out. She's said to still be asleep, but if she ever were to wake up it would be the end of the world. I don't know if she would have the charisma of being season 8's Big Bad, but I would like to see the boys fight an unstoppable, psychotic killing machine (rather than the smart, calculating monsters that the Leviathans were.)
I'd also like them to fight a sphinx, or a goblin, or a gorgon. Since monsters from different countries are converging onto the US (Lamlia, Okami) I'd like to see them fight a few more classical monsters.