Anything with Billy as Sherlock's son, and he's a result of past non-con, since Sherlock wouldn't have had a relationship, let alone decided to have a child of his own free will. After giving birth, Sherlock rejected him and Billy was adopted by other people (for some reason, the boy was named after Sherlock, who couldn't care less back then. Maybe it was Mycroft who chose the name and made sure to find good adoptive parents for his nephew. Maybe he's even kept in touch with the boy somehow).
Years later Sherlock meets Billy, who's so much like Sherlock himself, and it clicks. Please, a lot of angst and insecure Sherlock.

If it's an A/B/O verse, please Omega!Sherlock and Alpha!John, even though they don't have to be a couple (they can be in love nonetheless). Supportive John, please.