I’ve been thinking more and more on that line in the I love you trailer “what’s the worst thing…darkest secret” and I think it’s to John not Sherlock. John’s the one whose past we know nothing about and we apparently see a slip in his moral compass. Is it possible something happened in his army days? He ended up doing or covering up something messed up? What about Major Sholto who lead a team of CROWs into battle who all died? Did John have something to do with that? If so, it’s highly likely Culverton Smith was either involved or knows about it. Anyway, I think it’s definitely possible that we’re going to see John reveal his darkest secret, possibly in front of sherlock.
(This is by no means thought through, well put together, or all of my evidence, so feel free to add or dispute anything because it’s literally just my thoughts)