Squeeing because Sherlock is back!! No spoilers but I'll put it under a cut just in case.
So I enjoyed the episode, and I'm intrigued about where they go from here! But I think it's pretty clear that this episode is just the first act in a three-part series that are all connected. Just like in Series 3, I feel like we're not being given the full story for anything that's going on, so it seems sort of pointless to speculate heavily at this point. We just don't have enough information.
P.S. If you haven't seen it yet, there is an after-credits scene! Watch through the credits!