Um, I have another idea. :3 I'm going to type it down or else il lforget, so sorry for the double post. x3
Anyway, I was thinking that this could be kind of a part of the lead up to when they get into a relationship??
So it's night time and they're in they little base, maybe in the forbidden forest or something. And it's dark out. And John's having trouble with a spell and getting the hand movements right. So Sherlock comes up behind him and wraps his hand around John's, and leads him through it which ends up with them casting the spell together. And maybe it's the patronus charm? So it lights them up and kind of runs around them. 
I don't know what his patronus should be, though. xD Maybe a hedgehog, or a dog(because he's really loyal)? Like, a Golden Retriever or something?
But it's just an idea. :3)