The bug of dad!Zestial and mom!Eve bit me so hard... so maybe an au where both are possible?
I can see Roo and Zestiel collaborating to have their son, but they feel nervous having a child like theirs raised in hell, so Roo takes it upon herself to head to earth to have their son, inadvertently leaving behind the powers/name of Roo and becoming Eve again.
Eve and Alastor live as happily as they can im Jim Crow south, but Eve is taken early by a jealous stalker who wanted to marry her. She returns to Hell and becomes Roo again, and all she and Zestial can do is wait for the fate of their boy to be determined.
(Maybe Rosie, through the deal, hides Alastor's identity? Somehow keeps his soul from being known by both parents?? Idk)
Zestial genuinely loves both Roo and his son, and is incredibly happy to have both in Hell with him. Alastor fully believes his mom went to heaven. They don't know how to break it to him that the Root of all Evil was his mild-mannered mother.
Ooh, I love this! When Roo goes to earth she is Eve again - and like humanity came from africa, Eve was a black woman, so Alastor is still mixed race. She tells him his father was white, and a wealthy man who loved her but they couldn't be together and he died, but he ensured she was taken care of (Zestial was like 'trust me money is important you will need some') and Alastor grows up in that community and adores his mother and then she dies - and there is no justice so he kills her killer, and does his own thing.
The deal with Rosie means his soul is hidden from them...and Zestial was looking...for his DAUGHTER, because Alastor is a trans man here and Eve died before she could learn that - they both think their beloved child is in heaven, meanwhile Alastor thinks his beloved, kindly, motherly maman is in heaven.