After watching all episodes and spending too much time lamenting that there are none left to watch, I decided to join in the masses that try to deduce what's going to happen next.
One of the three "hints" given to the fans was wedding.
Now, interestingly many deduced that it is a reference to John and Mary being married in the actual novels. I do not think so; two reasons why I disagree with this notion.
1. Wedding - not marriage. It is true that John and Mary were married in the books, but their wedding itself never was part of a case. The hints given in the past directly referred to cases. Woman, Hound, Fall. All of those are in direct relation to their respective cases. John's wedding from the novels not. Why refer to some background facts given in the novel instead of the actual case? To me, wedding must be refering to another case. Especially since "Sign of the Four" was already implemented into "The Blind Banker".
2. Come on - wed off John? Wouldn't that completely disrupt the series' flow? Him moving out of Bakerstreet, no longer any Johnlock-bantering? While it seems perfectly fine for me to wed off John and novel-compliant, it seems unlikely that the directors would not stick to their previous principles.
Until now, they especially encouraged fan-squealing, slashing. BBC Sherlock virtually made Johnlock canon-compliant. The bantering, the casual mentions of homosexuality are put there as a big thumbs up for fans. Wedding off John would be like.. suddenly saying, "oh no, sorry there, we were just messing with you and laughing our asses off watching you ship the characters".
Honestly, I would be disappointed if "wedding" simply referred to John and Mary. It's not the fact that my inner fan screams Johnlock - it would just be too predictable. BBC Sherlock and predictable? No. I'd be severely disappointed, probably like most of the fans.
On the other hand, and that IS worrying, the directors once said that they'd be interested in the change of relationship between John and Sherlock that would arise due to John marrying.
Of course you can see that from the other direction as well: why not marry off Sherlock? A twist not many would anticipate. There exists one case where he becomes engaged to a housemaid. Problem here is the same I pointed out in the beginning: it doesn't exactly represent the case, nor does the wedding happen in the books. The engagement is broken off.
Another wedding that occurs in the books is Irene's. Still, I do not consider this likely. Same reasons I don't think we'll see John being married off. Not case-relevant. Wedding probably refers to some case where it is relevant, essential.
So what could wedding refer to?
There are several cases that revolve around weddings - like the case of the bride suddenly being left by her fiancé that had actually been her step-father in disguise. A short story, so unlikely to have made the "cut", still an example.
Personally, I expect a sham wedding. It doesn't matter who marries whom - for all I care, Sherlock and Irene can hit it off. I just don't think there will be an actual change, it probably is a way to flush out some criminal or something similiar to that.
