Doctor Ruth - idle speculation

So... Chris Chibnall says that Dr Ruth is categorically a Doctor. RTD lied. Moffat lied.
Ergo, we can't necessarily read anything into what Chibs says.
The press has been trumpeting the first black female Doctor, so for Chibnall to backtrack on this would be a huge mistake.
Ergo, she is a Doctor.
The big questions is: what kind of a Doctor?
Before we look into this, let's look at the story so far:

• The First Doctor is the First Doctor. The Time Lords said so in The Three Doctors, and subsequent stories confirmed this (such as the Eleventh believing he was the last, but getting a new regeneration cycle at the conclusion to Time of the Doctor).
• The Second Doctor.
• The Third Doctor.
• The Fourth Doctor. Things go slightly awry here, as The Brain of Morbius seems to imply that there were other, male, Doctors prior to the First Doctor - but only 8 faces are seen. This might be the first clue as to Dr Ruth's origins.
• The Fifth Doctor.
• The Sixth Doctor.
• The Seventh Doctor.
• The Eighth Doctor.
• The War Doctor - so actually the Ninth Doctor.
• The Ninth Doctor - who's technically the Tenth Doctor.
• The Tenth Doctor - who's technically the Eleventh Doctor.
• The Metacrisis Doctor - who's technically the Twelfth Doctor.
• The Valeyard - who is said to come between the twelfth "and final" incarnation of the Doctor (in this regeneration cycle?). That "and final" opens a whole can of worms, as he could come from anywhere after the twelfth - which is actually the Metacrisis Doctor - so another possibility for Dr Ruth's origins.
• The Eleventh Doctor - who has now become the Thirteenth Doctor.

And relax...

• The Twelfth Doctor is actually the first of the new Doctors - or the Fourteenth Doctor,
• And the Thirteenth Doctor is the second of the new Doctors - or the Fifteenth Doctor in old money.

Three possibilities are apparent from what is already canon. The first is what we have been hearing about lately, rumour-wise, and there really were Doctors before the First. Philip Hinchcliffe and Robert Holmes certainly wanted to imply this, and everyone adores them, but present day fans might not be so accepting...

The Cardiff DWAS local group request a meeting with Chris Chibnall...

Now Hinchcliffe and Holmes at that time weren't arguing for a whole new regeneration cycle - that only came later when they were running out of Doctors thanks to Holmes' own self imposed "12 Regenerations" limit. They simply thought that the Doctor could have lived a lot longer than we had seen on TV, and there were some prior to William Hartnell.

If Chibnall is going to go down this road to damnation, then he could argue that he is merely following in the footsteps of giants - just as he has been doing for all of this series so far with its blatant RTD and Moffat steals... (God forbid anyone should ever accuse Chris Chibnall of originality).

The second opportunity to slip Dr Ruth in comes with the Valeyard. What if she is what used to be he? The Valeyard is an intermediate Doctor, but a Doctor nonetheless - and potentially one that can regenerate. Dr Ruth is rather aggressive and violent, which is more fitting for someone like the Valeyard. 

The third opportunity is the Metacrisis Doctor. What if he wasn't as human as everyone thought, and regenerated into Dr Ruth? This would tie in with the other main speculation that Dr Ruth derives from an alternative universe, as we know that the Metacrisis Doctor was sent off to Pete's World with Rose. In The Rise of the Cybermen, the Doctor seemed to be saying that the Time Lords transcended these dimensional barriers - so weren't bound by them, and the Doctor is the only UNIT member not to have an evil doppelganger in Inferno. An alternative universe Doctor is still a Doctor.

There is still one further option, which is a little bit "out there" - so naturally I think it might be possible...

What if it isn't Dr Ruth who's the alternative one? What if it's Thirteen herself?

What if, when the Doctor regenerated at some point, the Doctor(s) we saw were the alternative universe ones, split off in some way, and the real Doctor who started with the First, as played by William Hartnell, went on to become Dr Ruth?...

People commented at the time that there didn't seem to be much character development between Missy and the Sacha Dhawan Master - so maybe this is an alternative Master. Gallifrey might still exist in our universe and only be destroyed in the alternative one - or vice versa.

Who knows, the Doctor's next regeneration might be a realignment, when we get the Doctor we were supposed to get - rather than the one we got...