I'm thinking the mystery woman has to be the White Guardian. She's obviously using him as her intermediary.
Bear with me as I ramble on a little
. I always wondered since the end of season 4 where Davros and the full might of the Daleks was used, how could you top that. Each season always tried to top the last leaving the question of for the next season, how can anything be more threatening?
My mind for a big bad villain for a 5th season is one of three possibilities.
1 The Great Intelligence. He appeared in the Patrick Troughtons era in 2 stories, The Abominable Snowmen and The Web of Fear. In the Web of Fear, he was shown to have the power to stop the Tardis in space. He is powerful enough to be a threat of a larger magnitude but I know very little about him or if he made any appearances in stories beyond the classic series.
2 The Celestial Toymaker. Appeared in the first doctor's run in a story of the same name. He is considered a guardian in later years on par with the other guardians and pulled the Tardis into a dimension of his own creation and he is immortal. He could also provide a greater threat then the Daleks and could have manipulated the events of the first four seasons and Time War in some grand game all befitting his style.
3 The Black Guardian. First appeared with The Keys of Time (The Ribos Operation,The Pirate Planet,The Stones of Blood,The Androids of Tara,The Power of Kroll, and The Armageddon Factor) and later in The Guardian Trilogy:(Mawdryn Undead, Terminus,and Enlightenment). The Doctor denied him both the Keys to Time and Enlightenment and drew the Guardians wrath.
The Guardian would also be aware of the events in The Trial of a Timelord and the Valeyard, the doctors possible bitter final regeneration. Quoting from the official site on the Valeyard:
The Valeyard was promised the Doctor's remaining incarnations by the High Council. He is an amalgamation of all the Doctor's evil, and is between the Doctor's 12th and final [13th] incarnation. This distinction would only be made if the Valeyard is different from the Doctor's 12th and 13th selves. Thus, he seems to be of the same nature as Cho-Je (Planet of the Spiders) or the Watcher (Logopolis), a projection of the Doctor's future self, one which might not be like what the 13th incarnation actually turns out to be.
Being aware of this possibility, the Black Guardian might want to get his revenge by turning the Doctor into the Valeyard.
Of these three villains, The Black Guardian has the best prospect, in my mind, to be the 4th season topper. He has the power to have manipulated the Time War and events in the 4 seasons we saw unfold. This would also mean the White Guardian would be involved as well. Take a look at the episodes with this in mind and you could see their possible involvement. Rose could have been placed in the Doctors path to try and save him from the fate of becoming the Valeyard by the White Guardian. How could she have survived so long with the power of the heart of the Tardis in her to destroy the Daleks. On the other side Rose being taken away at the end of the next season would fit the mold of the Black Guardian and hurting the Doctor by taking her away.
(Yes I did give a warning about my rambling
) I could list more incidents that could be associated with the Guardians but these few are enough to validate the possibility. Having seen the End of Time part 1, and seeing the speculation about the White Guardian and Timothy Dalton possibly being The Black Guardian. (Of course there is the possibility that we have the two of them backwards. Could the Black Guardian be the lady dressed in white trying to further his plans of creating the Valeyard and The White Guardian be the Lord High President. Remember he did impersonate the White Guardian at the end of the Key to Time storyline.) I have to wonder if my imagination on the possibility of topping the Daleks of season 4 are coming true. Could Russell T Davies be actually bringing this to the series and for some strange reason I had a similar idea prior to it actually happening?
I will be honestly floored if my imagined idea actually happens in some fashion. Sure he is not a villain used in a 12 episode season and then confronted in a two part epic season ender but the possibility is here and about to be revealed. I can take some comfort that I told a few of my friends who enjoy the show about this idea after the end of season 4.
I will now end my rambling as I just wanted to post this for your consideration as to what might be happening.