Moffat recently did an interview with Entertainment Weekly about the upcoming (and highly anticipated!) season four of Sherlock. Here are a few of the highlights, or you can read the interview in full here. Slight spoilers if you haven't watched the first three seasons.
First up, apparently Moffat seems surprised that fans that have overanalyzed the show over multiple re-watchings haven't seemed to ask the right questions about the upcoming stories.
There are answers coming to questions which nobody has asked. There's one thing that no one has really brought up… We've actually set up something, I think—[co-creator Mark Gatiss] and me, we're very exultant about a little thing we've set up that no one is talking about. It's not that we're being clever. We never know. Sometimes people go mad for one thing we think is quite trivial and completely ignore something we think is standing right in front of you.
[Spoiler Speculations] Maybe it has something do with why Redbeard was such a pressure point for Sherlock? Something to do with his involvement with Irene Adler? Is Watson going to face consequences of killing the cabbie in A Study in Pink? Or Sherlock the consequences of what happened with Magnuson?
Moffat states that the scripts for the season haven't been written yet, but he does give us the general tone of what the arc of season four will be:
[Season 4] is going to be… I suppose you'd say… consequences. It's consequences. Chickens come to roost. It's dark in some ways—obviously it's great fun and a Sherlock Holmes romp and all that—but there's a sense of… things… coming back to bite you. It's not a safe, sensible way to live. It's hilarious and exhilarating some days, but some days it's going to be bloody frightening.
In the past while each episode stood on their own very well, there were still very clear overarching plots, between Moriarty and Mary, they also had their overarching theme, which Moffatt describes, "The first series was all about the beginning of their friendship. Second about the formative stages, the love and fear and loss and all that. The third was good days, me and my pal and my pal's wife. Those are golden days."
He also claims that he would be in favor of "pleasing the fans" with a Sherlock/Doctor Who crossover event.
My instinct—and this is probably from years of doing Doctor Who—is I'm just such a tart. If people want to, we should give it to them. But I got persuaded by Mark, Benedict, [executive producer Sue Vertue] and Martin saying, "Look, it will never be as good as they think it's going to be," and then I say, "Yes, but we'll just bang it out and make it as good." "Yeah, but you can't give everybody everything they want all the time." I'm in the camp of giving them everything they want. But I think they're sane and right and I'm just a tart.
He concludes with saying that the way they do the series now, with three 90-minute episodes, is likely going to remain the format indefinitely, stating that it keeps the quality high and the actors invested.