Possible mpreg, omega!verse, or (past) Sherlock/ofc. John finds out (without warning) that Sherlock has a daughter when the girl, about 9 years old, comes for a very short day visit. He's never had custody of her not just because of his drug use but because he's never been mentally stable enough to take care of her properly. Even with her present, he's distant and critical, never remembers to buy food, and has more than once simply left her alone at home to go chase a case, even after he got off the drugs. She was very quickly taken from his custody by Mycroft and he feels no resentment about it. None of it's done maliciously, he just lacks the empathy and instinct to truly care for her. Which doesn't help with the whole sociopath label he occasionally trots out.

Mycroft has taken care of her needs via nannies and boarding schools since she was an infant. She's never been neglected or abused. She just doesn't have either parent there for her. Sherlock doesn't seem to care, maybe looks at her as if she's an alien being that in bizarre way has his DNA. Her mother/father/other parent is absent, maybe a drug addict that's long since left her behind, maybe dead.

If a romantic pairing is needed, I'd prefer it to be Sherlock/John. John could help her understand her father, why he seems incapable of loving her, and help Sherlock not be afraid to acknowledge that he feels some love for his daughter.