NOBODY IN THE BSC IS ALLOWED TO MARRY ANYBODY ELSE'S DAD, I FORBID IT.

It's gotta be a fandom where somebody marrying their friend's dad is at least... conceptually intriguing, and there's also a very strong sense of place, or there's not really any hook. Both of those apply to BSC except NOPE.

Merlin BBC? Merlin hooks up with Uther at Arthur's wedding but it turns out he was possessed by the dragon at the time?