On Dec 30th, the official Sherlock YouTube released this video and during it a game of hangman is played, a very fitting game for what is revealed in it. Here it is:

It spells out "Frank Hudson" who was Mrs Hudson's husband before Sherlock ensured his death by hanging in Florida. Frank was secretive, led a double life, and was a criminal who ran a drug cartel and eventually blew someone's head off. Their marriage was a whirlwind and Mrs Hudson says she never thought it was actually true love. He was also adulterous with several women.
As you can tell, their relationship has many, many similarities with John and Mary, emphasised with Mrs Hudson talking about this to John before his own wedding. I can only assume from this that we are going to see more of these parallels come to light and perhaps learn more about Mrs Hudson.
Mrs Hudson says that her husband was "just the same" when Sherlock is rushing around in ASiP, Mrs Hudson's best friend left her wedding early, Mrs Hudson has a bad hip that she directly points out is a parallel to John's bad leg, Mrs Hudson's neighbour has a married gay couple who will eventually mirror Sherlock and John (because Mrs Turner and Mrs Hudson were the same person in ACD's stories).
Mrs Hudson is a John mirror, which means Frank is mirrored in Mary, and we are going to see more of this play out I am certain. Frank was hanged for his crimes as was ensured by Sherlock, and Sherlock is also the friend who left early, so this could get very messy and painful.
Brace yourselves. S4 is imminent.
Some extra info:
In Jan 2014, this Q&A happened. This is one of the questions, and look at the answer...

Frank Hudson was also the name of Mrs Hudson's husband in The Veiled Detective, a novel by David Stuart Davies, which you can read about here. It was originally published in 2004.
Comments (20)
This is awesome !
I bet when mrs. Hudson was younger she work in the same field of work as mary and she has a bad hip because of getting shot when she was younger.
Wow. This is mind blowing!
... Wha... Wait wha... my brain just imploded. Also, Mrs. Turner and Mrs. Hudson are the same in the show too. In one of the comments in John's blog, Mrs. Turner says "This is Mrs. Hudson by the way."
Yes, she does, it's a big joke because of Conan Doyle mixing their names up.
This......shoocked me to the core.....