I understand Davies isn't going to retcon the Timeless Child arc, but in a few years time, a future writer/showrunner could use my theory.
We know Ruth comes before the 13th Doctor, and 2 popular theories include her being pre-Hartnell, and the other being her being in between the second and third Doctor. However, both of these are flawed as Ruth's TARDIS is stuck as a police box, and the first Doctor was the one who first realised this, while Ruth being between 2 and 3 doesn't work as it would mean that David Tennant wouldn't be able to regenerate into Matt Smith since his aborted regeneration in The Stolen Earth was supposed to be the last one, rather than 11's regeneration in Time of The Doctor.
So, here is my idea. We never saw the old 11th Doctor deaging back to his younger self at the end of Time of The Doctor, so this could very well have been where the Fugitive Doctor appeared. She might have flew the TARDIS out of Trenzalore, lived her secret life then regenerated. The Time Lords never disclosed how many extra regenerations the Doctor got in Hell Bent, so who knows if Ruth had regenerated several times before regenerating back into Matt Smith's Doctor, returning to Clara where he would become Peter Capaldi's Doctor. This would also mean that the Curator's promise of revisiting old faces in the 50th anniversary special would be fulfilled a lot sooner!
Before you ask, Ruth could have changed the TARDIS interior to the one seen in Series 12 back to 11's before she returned to Clara. As for 13 not recognising Ruth, the Doctor could always have lost his/her memory after Ruth's incarnation died.
Any flaws you find in this idea are welcome to be shared so I can improve the idea.