She could be Romana , but somehow that feels off, i have predicted Romana was one of the Docs wives, happening between series, The Doctor's mother is a very possible thing, she could be any number oif things, she could even be an avatar of the time lord matrix, but the real question is..who is timothy dalton?
is he...
1.) the Master in a future incarnation?
2) the valyard? (the doc's possible final regeneration seen in " trial of a timelord")
3.) Borusa ? (somehow freed from stone prison ala five doctors)
I don't think Gallifrey, as it was depicted in Part 2's trailer, is back in a sense. It is more likely a flashback of sorts, depicting what the Time Lords were like in the final days of the Time War. However, we see them talking about the final days of Earth and Gallifrey, indicating that their point of view took place somewhere in the past. Timothy Dalton's Time Lord may not be either the Valyard or what John Simm's Master would look like in the future, but he may have been in 'cohorts' with the Master (Anthony Ainley era) in some way during Gallifrey's last days.
They may have been planning a takeover of universal proportions, not that it wouldn't be difficult for a Time Lord mind you.
I'm using Anthony Ainley's Master as an example because after that, it is canon that his physical body was killed while on trial on Skaro but that his essence had survived as a slug-like creature and eventually took over the body of Bruce in the TV movie, resulting in Eric Roberts Master, and since Eric Roberts Master never even made it back to Gallifrey alive, this is a most likely theory.
We know that Derek Jacobi's Master was resurrected from the Eye Of Harmony in the Doctor's TARDIS, which was constantly connected to the Untempered Schism on Gallifrey, giving the other Time Lords direct access to the Master's atomic signature, allowing them to ressuerect him as a child with new regenerations, planning for him to fight in the Time War. But we also know, that Derek Jacobi's Master escaped Gallifrey because of the impending Time War and used the Chameleon Arch to store his Time Lord consciousness in a fob watch.
By doing this, he made it impossible for the other Time Lords to find him, meaning its possible that Timothy Dalton's Time Lord would not be able to find him either, and thus, meaning that he and John Simm's Master never met up again because Timothy Dalton's Time Lord character would have died in the Time War with the other Time Lords.
