When I saw the word wedding, my mind immediately jumped to “The Adventure of the Noble Bachelor,” in which Holmes tracks down a runaway bride. This seems like a likely candidate for several reasons:
1. Hatty Doran, the aforementioned bride, is a gorgeous, plucky, free-spirited woman, and we all know how much Moffat likes those.
2. Moffat is a notorious troll. I’m sure he deliberately chose the word that would get the biggest reaction, and I wouldn’t be surprised if he were deliberately trying to mislead us.
3. “The Noble Bachelor” would translate to a modern setting quite nicely. It’s basically about a celebrity wedding gone awry, and celebrity weddings are as popular now as ever.
4. Every episode title so far has referred to the main case being investigated in that episode. So even if John does get married in Series 3, I doubt the episode will actually be called “John’s Wedding.”
TL;DR: Calm down, everyone. This doesn’t necessarily mean John is getting married.