After the end, they buy a house.
Castiel finds it for her and leaves her there with Kevin, then goes with the Winchesters to help clean up the few remaining demons that also managed to avoid being sucked into Hell. Kevin needs protecting, he says, and she does it, for lack of anything else to do.
She takes a job as a school nurse at the local High School, taking kid's temperatures and handing out feminine hygiene products to embarrassed students. It's boring, but it keeps her away from Kevin during the day and gives them enough money to keep them in food and beer, so she doesn't complain. She could be trapped in Hell right now with Crowley, after all.
She thinks about maybe smoking out and finding a new body when the other demons are dead. Disappearing to the other side of the world where the Winchesters could never find her and taking up her previous existence, killing and stealing and body-hopping until the end of time. She files the thought away as quickly as it comes up, most days, knowing that even with demons gone there are still hunters out there.
A year and a half into pretending to be human, Castiel appears in her living room to tell her she is the last demon on Earth. She tells him me makes a better door than a window and shoves him away from her television screen.
He stays with her, but the Winchesters move on. Even with demons gone, there's always something that goes bump in the night for them to hunt.
They cope. Meg goes to work and paints scenes from Hell into the long hours of the night. Castiel tends to the garden and cleans the house and watches over Kevin. Kevin tries to get his life together, takes college classes online, and by the end of the third year, can actually remember to eat without Meg or Cas reminding him. At night, when Kevin is asleep and Meg puts down her paintbrush, she takes Castiel into her bed and laughs when she realizes what an abomination their 'relationship' is to everyone else.
The Winchesters stop by whenever they're in the area, to glare at her and update Castiel and Kevin on their progress ridding the world of evil. Castiel holds her hand whenever they drive off, staring after them almost wistfully. When she tells him he could go with them, he grips her hand harder and tells her that her house is as good a place to stay as any other.
They pretend to be married, for appearances. On the weekends, she'll sometimes bundle Castiel and Kevin into their car and take them somewhere, just to be out of the house. She takes them to zoos and aquariums and makes Castiel watch It's a Wonderful Life, just to see his face when he finally understands why she calls him Clarence. The teachers at school ask her when she and Castiel are planning on having children, and she always rolls her eyes at them, but says "maybe next year" and gives them fake, hopeful smiles. They wear fake rings and make up a history, sprinkled with truth, about how they're all alone out there because neither of their families approve of their relationship.
It hits Meg as she sits at her easel that this can't go on forever. She and Castiel have another few years, five tops, before people start to notice that they don't age and they have to move on. The Winchesters and Kevin will die and one day Castiel will leave her for Heaven and she'll be alone, the last demon on Earth, with nothing left to fight for.
She sets down her brush and stares at her painting, herself on the rack when she'd first gone to Hell, and thinks about cutting and running right then. She could kill Kevin and Castiel and smoke out, leaving behind her body for the Winchesters to find.
The thought passes as quickly as it comes, and she takes up her brush again. Still, she wonders how long she can play human all by her lonesome, with no cause to guide her, before somebody catches on and some other hunter hurls her back into the Pit to be Crowley's plaything.
She puts down her brush again and leaves her stool, moving through the house to stand on the back porch. Kevin sits with a thick book on physics and a glass of lemonade, glancing up now and again to watch Castiel pull another weed from their flowers.
She pulls up a chair next to Kevin and steals his lemonade. Cas waves at her from the garden and she smiles back at him.
One day she will be alone, truly, and they will all leave her, even Castiel. She will spend eternity walking the world on her own, restless for a cause, until humanity falls to dust.
Still, life goes on. Kevin reads and she paints and Castiel hands her flowers from his garden. It's only forty years later, with the Winchesters gone and Kevin posing as her father while they stay forever-young in a new town, that she tells him she loves him.
He doesn't say it back, but hands her a bouquet of daisies from their newest garden. Her future stretches, bleak, in front of her. She takes the daisies anyway and smiles at him.
It doesn't matter that one day he will leave her, and she will be alone. For now, playing human is enough.