BEWARE OF SOME BRIEF SPOILERS
As the countdown to the Elementary season finale begins, I've come up with a theory concerning Irene Adler and Moriarty that sets up a radical direction for the show:
I believe Irene Adler and Moriarty are one and the same.
I think Moriarty is an alias Irene used to commit her crimes through her operatives. She created the Moriarty alias not only to cover up her true identity but to fool others into thinking she is a he.
Irene got involved with Sherlock initially to get to know the man who has been poking his nose into some of her crimes and then to remove him as a threat to her plans. But the more she got to know Holmes, the more she was less inclined to kill him which is why she told one of her assassins to forget about the plan to kill Holmes and make his death look like an accidental overdose.
However, Irene knew that this relationship with Holmes couldn't last forever, him being a criminal investigator and her being a criminal mastermind. There's no future in them being together so Irene faked her death and made it look like her alter ego killed her. Over the years, Irene has been keeping an eye on Holmes, him being her greatest object of curiosity.
I think once Holmes finds out all of this, it will push him over the edge into a relapse.
If my theory is correct, this revelation will make the Holmes-Adler relationship more complicated and twisted than the original Holmes-Adler relationship.