As always... ***SPOILER ALERT***
I just discovered that there is a deleted scene from the final episode of Season 3. An interesting detail from this scene is that after Sherlock was shot, a single rose was sent to his hospital. The note had a weird picture of an eye and the letter W on it.
That begs the question: Who is W? C.A.M. denies being the sender in the deleted scene. Dr. and/or Mrs. John Watson would have signed their full name(s). Who then is the mystery person?
The first possibility is that Gatiss/Moffat threw it in to hint at what is to come next season, It would not be out of character for them to throw in a sly foreshadow to future events.
The second possibility is that the flower is from Bill Wiggins. It appears the young homeless man is now on team Sherlock for a permanent basis, so this could be his way of saying "Get well, boss" or "Please, hire me."
The third possibility is that the W stands for The Woman. This is intriguing for those who would love to see that character return to the show. It is hard to believe she would allow Sherlock to be shot without some note of affection and concern, but would she risk the exposure? The other hard part to swallow about this theory is the idea of The Woman calling herself "The Woman." It seems unlikely she would use that nickname for herself. However, it is possible her new identity has W for an initial.
W could be anyone, and when you consider the possibility that the W might just be an upside down M, the possibilities increase.
Now the other image becomes a little more understandable. It is an eye on a weird shape. Is Mary, Magnussen, or Mycroft trying to say, "I've got my eye on you."? The eye does seem remarkably similar to the "All-Seeing eye."
The shape looks a lot like a chess piece. A pawn, perhaps? Could this be a reference to the ever present chess game between Moriarty? This may be the best theory, especially considering the fact that this is from a deleted scene. Gatiss/Moffat may have deleted it to make the reveal about Moriarty more shocking.
With this show, wondering is always more entertaining than actually finding out.