Before the show was revived, BBC said the audios were canon, but after the show was revived RTD said (and thus BBC said) that the show considers the previous TV show, and TV American Movie canon, and the rest was up for grabs. Obviously the stories that appealed the most can get written into the show (Human Nature, for example) and some things seem inevitable - so many stories had Romana as President and being corrupted by her battles against the Daleks, Rassilon reappearing and being far more corrupted than Romana, who was at least trying to do right, .. that sort of thing seems to bleed over into all the different formats.
I usually just use BBC's notes on various episodes as what happened, and the rest, as Moffat so eloquently put it, is all 'wibbley wobbley'
... or, well, interesting to think about, but by no means certain - but then when traveling with a timelord who can help history unfold in lots of different ways then it did before, what is?
According to mrfranklin and M'reen the only thing we can consider canon is the classic serial, the Movie, and the series (unless BBC decides to change its mind yet again about the audio adventures. I'll have to ask the Cadmium 2 trio to see if they have any verifiable proof.) I am sorry I failed to respond to Ace as a possibility. I should have replied but considering I know nothing of her fate after Survival, I doubt I could have really made a good and thought out opinion on this.
Another thing that makes the idea of Ace as Woman in White seem unlikely to me is the fact that the only Companion we've ever see return in NuWho is Sarah Jane, the single most popular Companion of all time. I just don't see them bringing back all sorts of classic Companions without a really good build-up...
I think that any decision to bring another classic companion back might have been kept under the radar. What greater surprise for the fans could we have had with a surprise return. The fact that the Doctor never said anything does make me lean away from the Lady in White being Ace. What could have happened to Ace that he would have kept quiet like that?
I'm sure at some point Moffat will find a way to let the Timelords be an influence again, somehow. They were always a powerful, pompous, dodgy group that made the Doctor have to be very very clever if he wanted to get away intact. Always a nice change of pace from 'There is no higher power, I have the last say' Doctor we've been dealing with.
Let's consider something from the season 4 Davros return. Quoting from the Synopsis on Wikipedia: http://en.wikipedia.org/wiki/The_Stolen_Earth
Davros, who was thought to have perished at the beginning of the Time War, was saved by Dalek Caan, who entered the conflict after performing an emergency temporal shift. The power needed to enter the Time War—which is "time-locked", preventing time-travellers entering the conflict—caused Caan to become precognitive, but at the cost of his sanity.
Dalek Caan somehow found a way to enter the Time War and perhaps that created a tiny hole or break in this time-lock. This sounds like the perfect culprit for any Time Lord, or anything else involved on any side of the conflict, to make an appearance in the future.
Unlike my thoughts on the Guardians being involved in the End of Time, I believe this is a strong and highly likely reason to be used should such events transpire in the series I feel very comfortable in saying this