Well I've definitely already heard from some others that were (even massively?) disappointed by this Christmas special, using the "it was all a dream" crutch. But no, I've said for a long time now, you CAN get away with 'it was all a dream' as the ultimate answer for your story, if it's not just done for the sake of itself, and the writers had just boxed themselves into a corner. But here it was used in a very specific way, it was all handled in a proper context, and I was thoroughly entertained by it. I'd had a sneaking suspicion for a long time now that despite Moffat's remarks to the contrary (seriously, do fans not realize how much he's just straight up lying in interviews constantly when talking about either Sherlock or Doctor Who?), it was NOT going to be just entirely this fun little one-off special. That "oh wouldn't it be cool if we could just once use our actors but playing in the original time period?" was not going to be that simple and straight forward. I didn't know HOW exactly it would be, but I was confident it would be ultimately tied to the series proper in some way, and I mean beyond just cheeky dialogue references.
And in fact one theory I've already heard by several people that's pretty damn interesting, is a play on Sherlock insisting to John that "it's never twins!" But remember, twins doesn't always mean the same gender. That what was going on in the 1880s story was an allusion to the fact that Janine is Moriarty's sister, and has been aligned with him for the entire show. That yea, they stated unequivocally that Moriarty IS DEAD, but that doesn't mean he couldn't have had a shit ton of plans set to be pushed into motion by others in his absence. And that Janine could be an integral part of that. They do have a general enough sort of a resemblance, and they're both Irish.
Alright and this is totally aside from the Abominable Bride special episode here, but when doing some reading earlier today I came across this older theory. Although I think the evidence already shows this is absolutely true. That, if you remember the woman who was the jury foreman for Moriarty's trial in 2x03 "The Reichenbach Fall", her kids that she saw on the hotel room TV were being threatened to insure she'd help get him aquitted... well that was the very same family that the cab driver from 1x01 "A Study In Pink" was poisoning people on behalf of Moriarty to protect and make them as much money as possible.
http://writemeastoryofsolitude.tumbl...ies-in-asip-or