What if there was a 3rd person that rides along with Dean and Sam?

Back story: it's Bella Swan. But tougher thanks to John and Dean. They've toughened her up! Now she hunts with Dean and his brother Sam. Dean calls her baby girl, she's his best friend and he's hers. She and Sam fall for each other.

What if there was a way to stop Dean from going to Hell? What if Lilith had a Vessel?