Set in a world where the supernatural does not exist. Cass is a salaryman and Fandango is a sex worker. They meet when he is hired as a prank for her 40th birthday and hit it off despite the circumstances.
The AU I have spent the most time thinking about! I like imagining the casual domesticity. Also since everyone is mortal there's much angst to be mined from who dies when. There's sort of a loose branching timeline in my mind, I would love to draw it out someday.
In all scenarios I've considered, Cass dies first. She suffers a heart attack at 58 with two outcomes: she lives or she dies. If she lives things continue on. If she dies, the family in chaos. Fandango is ravaged by grief and becomes depressed and suicidal. Cyrus has to postpone going to college to come home and take care of his siblings while his father is incapacitated. We have two options here as well: Fandango lives or Fandango dies.
If Fandango lives it is only because he begins an romantic and sexual relationship with his son Cyrus. This would likely take the form of Cyrus initiating, perhaps even by force. He instinctively knows his father needs someone else to carry on, and he is that someone. They try to keep this secret from the family, but it's difficult. It hurts not being able to kiss and hold hands in front of everyone else. Problems arise when their secret is discovered. The youngest Martin is too young to care, the twins have their own incestuous thing going on so they don't care, however Maeve takes it extremely hard. She thinks its disgusting and her father and brother are disgusting for doing such a thing with each other. In a bad version Cyrus tells her dad needs this to stay alive, and she tells him she'd rather he be dead. Fandango either overhears this or she tells him to his face. Obviously this does not hit well in his mental state and he attempts to kill himself immediately.