For Billy and Briar, I'd think Peaky Blinders. And Billy could be Tommy's twin or something, and Briar could be a Gypsy seer or something. And they'd marry because of an alliance.
Then Teen Wolves, Briar can be a Hale and Billy an Argent.
The mentalist
Castle, where briar is the daughter of castle and she actually has a good relationship with her dad. And instead of being a serial killer here, she's a bit bored (not entirely) with being a straight killer she shakes it up a bit, she becomes a state mercenary and she has a lot of assignments.
Then John Wick, because why not.
Once upon a time, and she's the Evil Queen's daughter.
Scream Queens, and she and Billy find this as a comedy.
Maybe twilight, but they're not vampires, they curl their lips for the sparkly leech fairies.
Resident Evil
These are just the top of my head.