I know that's a weird title, and this actually refers to someone who hasn't shown up in the BBC series.
But, in Doyle canon, there is one young inspector, Stanley Hopkins, who is something of a Sherlock Holmes fanboy. In a proper Victorian way, of course, but where other inspectors show distrust or jealousy, Hopkins has only admiration. Readers and fanfic writers who are into slash like to interpret that he has a crush.
So it made me wonder how John-of-BBC would feel if there was someone else admiring Sherlock that much and wanting to work with him and learn from him (and in today's British television the guy could even be openly gay.) I wonder if it would be the equivalent of how Sherlock felt about Sholto?
I guess you could say Molly is the closest thing we've seen to this, and John doesn't seem to have a particular problem with her, but then, maybe her being female makes it not the same as his relationship with Sherlock?