I really doubt that the wedding mentioned in our three clues today is going to be between John Watson and Mary Morstan. And I have a few reasons why.
Their marriage isn’t the only marriage in Holmes canon. The other one comes from ‘The Adventure of the Noble Bachelor’, and deals with the disappearance of a bride from the reception of her wedding.
Mary Morstan is dead by the time Holmes returns in ACD canon. Watson and Morstan meet in the Sign of the Four and eventually marry, but by the time Holmes returns in ‘The Adventure of the Empty House’, Mary’s dead and buried.
We’ve already had our Mary Morstan character. That would be one Sarah Sawyer, from ‘The Blind Banker’. Sarah is probably John’s most serious relationship in the BBC verse (though it’s hard to tell, since we don’t know much about his other girlfriends), and they meet on a case that draws many parallels to ACD’s the Sign of the Four.
In the original stories, Watson has a second marriage. There are very few details (and by few I mean basically none) about this marriage, but it’s referenced a few times in later Holmes stories. So, if I’m right, and Sarah is our stand in for Mary, John might get another somewhat serious love interest, but as with Sarah, I doubt we’ll be seeing a marriage.
It really just wouldn’t fit, okay? Alright, this is a pretty awful reason, but here me out. If ‘Bow’ really does mean they’re ending Sherlock with this season, that means we only have 4.5 more hours of this show. 1.5 of those are going to be spent wrapping up how Sherlock survived, John and Sherlock’s reconciliation, how exactly Sherlock is going to reenter society after his time among the dead, and, since this episode is supposedly based in part on ‘The Empty House’, taking down Moran or some other high-level Moriarty lieutenant who still lingers on. This is a lot to handle while also introducing us to some new character that John’s taken up with, especially if they’re going to be married off in the next episode. I don’t think Moftiss would marry off one of their main characters to someone they haven’t had time to flesh out, at least to some degree.
so, uh, tl;dr—if this is our last season, I doubt they’re going to spend time introducing some romance for John. If they’re wrapping things up, they’re going to spend their time on our heroes. I hope.