What is the ultimate dark thing you can think of?
Sherlock being responsible for the death of John's child would be the darkest thing I can imagine... But as I can see no way for John, the audience or anyone else forgiving him that, it can't happen, can it?^^

Yes, but I wouldn't define that as being responsible. The person who made Sherlock decide would be the responsible one. Although this scenario would be pretty dark, too.

For me the darkest scenario (apart from Sherlock's or John's death) would be a permanent rift between them. Something apparently unforgivable. 

Yes. But in TEH it took Sherlock … what was it? One day? … to get John's forgiveness. And it could be the other way round as well, John doing something unforgivable. 
Btw, my answer was purely hypothetical, I did not see anything in Setlock to make me believe this will happen. 

I don't think they would make Sherlock kill John permanently, or make a permanent rift between them.   But I wouldn't rule out that they could do it temporarily.  For instance, with John seemingly dead for a while, Sherlock apparently killing him, them becoming estranged.  This could be part of a plan, to protect themselves or somebody else, which is only revealed later.   It could even be a cliffhanger - judging from previous episodes (seeing Sherlock alive at the end of TRF) I don't think they'd do that, but they might want a very dark finale.  But I think they would still leave a get out clause, a clue that it wasn't final. 


The darkest thing ... I really don't think they'd do this, but I'm thinking back to ASIP and Sally Donovan implying that Sherlock might be a serial killer.  The darkest twist would be making him actually evil, I think.  But I don't think this would fit with what we've seen of his character, and I can't imagine it being part of Moftiss's vision.  (Although again, they could do it temporarily, making Sherlock appear evil, making it seem as if we've been fooled all along - that would not be at odds with their writing, and would certainly be something Sherlock would be prepared to do if absolutely necessary).             

No, I don't think so, but as you say, it was hinted at in TRF, with Moriarty appearing innocent, and Sherlock possibly inventing the cases (as Sally Donovan implied) and actually being the criminal himself.  So I suppose that's a theme that has persisted through two series, even if it's a minor theme and quickly resolved.  I do feel Magnussen is slightly different, in that although it's terrible and dark there is no attempted subterfuge there, at least as far as we know. 

I would be open to something very much darker and protracted along these lines as long as Sherlock is eventually shown as the "goodie" he is!   Now, that would be something which would test the relationship with John, as so far John has always managed to have faith in Sherlock ... it's never REALLY being tested and found wanting.   That would be something new. 

Hope he doesn't depart!

Rache wrote:

What is the ultimate dark thing you can think of?
Sherlock being responsible for the death of John's child would be the darkest thing I can imagine... But as I can see no way for John, the audience or anyone else forgiving him that, it can't happen, can it?^^

I cease to see how John and Sherlock could remain friends if that happened.
Same with Sherlock turning evil - it would irreparably and ireedemably destroy the whole series, IMHO:
 

Just my personal opinion: I think the child does matter far less than we think. She will not be the focus of the show, not even an episode. And I really do not believe that anything like this would happen. Not least because there is no precedence for this in Canon, neither for a Watson child, nor for anything bad happening to them. At SDCC the baby was almost non-existant, nothing more but an afterthought in the panel. 
 

I really like your way of thinking. I still find it telling that Victorian Mary who works for Mycroft and has a somewhat bumpy marriage is not pregnant.