My son got a remote-controlled helicopter for his birthday. I was doing dishes and turned to find it hovering at the back of my head. It almost immediately occurred to me that this was something Sherlock might do to John. It’s the kind of thing that would be introduced as comic—Mycroft got Sherlock a toy for Christmas! And Sherlock uses it to annoy John and Mrs Hudson (and possibly also to do some kind of study of mechanics or something) . . . But then later in the episode the helicopter would become necessarily useful in some way, like having a camera attached to it, or using it to deliver something . . . Or maybe whatever Sherlock was studying via helicopter will come into play as an applied science. Maybe he will have to fly an actual helicopter. That would be . . . over budget probably. But that’s how one structures these kinds of stories.
Meanwhile, I intend to keep the helicopter someplace higher than my son can reach. For now.
Mr Moffat, you have permission to steal this idea so long as you name the helicopter Pepper in my honor.