I want to do an extra scene series for each of the six episodes, based on fun extra bits that I've daydreamed about (see my Scandal and Hounds stories to see what I mean). I've run into difficulties with each of the other four.
My problem with Reichenbach is the tone. It's a very beautiful and dark episode, which is a long way from my writing style. While I'm happy to inject elements of humour in the other ones, it would feel slightly wrong to do so here.
I do have a couple of ideas, though, if you can handle the slightly angsty feel of it.
Moriarty wanted Sherlock destroyed, right? And you have given him the perfect ammunition.
John, I'm sorry.
Tell him, would you?
Mycroft watched as John walked away down the corridor.
"Tell him," he said again, quietly. He retrieved his phone from his breast pocket.
"I can't," Sherlock replied. "You know that."
"You can trust him. I think you can trust him more than any other living soul."
"It's not worth the risk.
"No, I've told you; it's too great a risk."
"Then I'll stand by your judgement in the matter."
"Good. Everything's in place, yes?"
"This evening then. I'll text you when."
Sherlock disconnected the call. He paced back and forth for a moment, and then sank slowly to the floor of Molly's lab. He sighed and waited.