(inspired by the film Concussion) John is invalided home from Afghanistan and he's depressed. The usual story. He can't get a job (he's not a doctor, just a soldier) and is having difficulty adjusting to civilian life. Eventually, he meets Mary (or another woman), falls in love, gets married and he's finally happy. He doesn't ever quite get much excitement in his life, but at least he got the perfect life he'd always thought he wanted. He and Mary have a child together, and as years pass John doesn't ever get a job. Mary is the breadwinner, she's (as in canon) a doctor/nurse and John is pretty much just a stay-at-home dad. He's incredibly bored with life and his marriage has hit a low point (hasn't had sex in awhile, Mary unusually distant all the time) so, fed up, he tries something new. John is bisexual (and insecure?), so he hires a prostitute/rentboy but it turns out to be a disaster, somehow. Someone, maybe a close friend of John's (Mike Stamford?) tells John he knows a man who runs a small-time prostitution ring and is seeking a 'mature' rentboy for his male clients. John is convinced after friend/Mike Stamford tells him he'll be getting paid quite a bit of cash. John is hesitant about the whole affair, but tries it out and finds that he loves being a rentboy, gets really into it, enjoys the sex, finally starts enjoying life again (and doesn't tell Mary. But Mary isn't stupid, eventually suspects something is going on).

How Sherlock comes into the picture is up to the author--is he on a case involving the prostitution ring and is undercover when he meets John? Is he coked off his tits when he hires John for a bit of fun? John falls for Sherlock and vice versa; Sherlock continues to hire John just so he can see him again.

Ultimately a Sherlock/John ending and shared custody of Baby!Watson between Mary and John would be great.