7. Mycroft Dies
Sad news, Mycroft fans. Many theorists believe that Sherlock's big brother might not make it through to the end of the series.
This theory dates back to The Abominable Bride. In the Victorian dream portion of that episode, Sherlock blackly jokes about how much longer an obese Mycroft has to left to live. Back in the real world, we catch a glimpse of Mycroft's personal journal. Something that jumps out is the mention of 'Vernet?'.
Some fans believe this is a reference to the fact that Mycroft is suffering from Vernet's Syndrome, a kind of brain tumour. On the other hand, it could be a nod to Vernet being Sherlock's mother's maiden name in the Conan Doyle books.
However, most fans lean towards the first explanation, due to the foreshadowing in the Victorian scenes and because Mycroft seems oddly sentimental about his brother here. He even says "I'll always be there for you." As one redditer commented, that is as fatal a phrase as a cop saying they are a few days from retirement in an action movie.
More recently, this theory has been fuelled by an enigmatic promo for the fourth season which sees Mycroft's umbrella sitting abandoned by the Baker Street fireplace.