The Big Bad of Season Six can be someone or a group of individuals bent on ridding the world of all things supernatural to protect the public from any more dangers from that world. That way, while Sam and Dean feel that they have to fight them because of the terrible things this group is doing in the achievement of its goal, they also ponder on the goal itself and if a life without supernatural creatures is actually a bad thing.
This anti-supernatural group can be something like the Initiative from Buffy the Vampire Slayer or a vigilante group composed of those who've lost loved ones to monsters in the past, some of which Sam and Dean have met before.
It's not as badass as Lucifer but it's something different for the brothers to fight aganist. Maybe the group could try to recruit one of the brothers or both of them into their cause because of their own bad history with supernatural beings like the Yellow-Eyed Demon killing their parents and Sam's girlfriend Jessica.
This anti-supernatural group can be something like the Initiative from Buffy the Vampire Slayer or a vigilante group composed of those who've lost loved ones to monsters in the past, some of which Sam and Dean have met before.
It's not as badass as Lucifer but it's something different for the brothers to fight aganist. Maybe the group could try to recruit one of the brothers or both of them into their cause because of their own bad history with supernatural beings like the Yellow-Eyed Demon killing their parents and Sam's girlfriend Jessica.
