Now this theory is very popular online. Missy is believed to be a shortened version of the word Mistress, which is the female form of Master. But would the Moff be THAT literal? The Master has been rumoured to appear in series eight for a long time, with Charles Dance as the name often associated with the role. We know that the Master is not adverse to playing the long game - after all, he wove the facade of Harold Saxon through the minds of humanity for eighteen months prior to taking over the Earth. So perhaps the Master is building an army from those the Doctor has killed, in order to inflict guilt upon his foe. As series three showed, the Master successfully infiltrated Earth without the Doctor knowing, so perhaps he is behind the promised land mystery. If Missy is not the Master, she could be somebody associated with the Master. We last saw the Master with Rassilon when the Lord President attempted to return Gallifrey to the universe, so it is plausible that the Master was sucked back into Gallifrey along with the President. Once there, he could have hooked up with a new Time Lady - Missy - to concoct some plan to fight his age-old foe. This theory is highly likely.
