The TARDIS may hold the answers to Doctor Who's Timeless Child mystery. Doctor Who's season 12 finally began to unravel the mystery of the Timeless Child, revealing the Doctor is older than she knew. She originates from another universe, discovered by the ancient Gallifreyan explorer Tecteun on some distant alien world. The Timeless Child became the base genetic code for all Time Lords, granting them the power of regeneration.
Like all retcons, the Timeless Child revelation is not without its continuity problems, but it fits surprisingly well with clues throughout the classic Doctor Who series. Showrunner Chris Chibnall has restored a sense of mystery to the Doctor's origin, and no doubt future seasons will see the Doctor learn more about where she comes from. Matters will be complicated by the fact her memories have been erased by the Time Lords, meaning she doesn't even know where to find the clues.
This kind of story is usually a grand quest, but oddly enough for the Doctor, answers may lie in the TARDIS itself. Rather than explore distant planets in search of answers, she rather needs to unravel the mysteries of the TARDIS itself — because the TARDIS appears to have a longer history than she knew as well.
The Doctor Appears To Have Met A Pre-Hartnell Incarnation
An earlier episode of Doctor Who season 12, "Fugitive of the Judoon," introduced a "new" incarnation of the Doctor. Played by Jo Martin, this Doctor had hidden herself on Earth, using a primitive version of the Chameleon Arch to pretend to be a human. This mystery Doctor had been working for the Time Lords, and had gone rogue, hiding herself away. The episode deliberately avoided confirming where this Doctor fits in the timeline, but she appears to be a pre-Hartnell incarnation. The interior of her TARDIS bears a close resemblance to Hartnell's, she has Hartnell's habit of referring to the TARDIS as her "ship," and she doesn't know what a sonic screwdriver is. This is particularly important, given the sonic screwdriver was invented by Patrick Troughton's Doctor. Supporting this, Jo Martin has insisted she shouldn't really be considered a "new" Doctor. "I'm a Doctor," she observed, careful to be precise with her language.
If Martin is a pre-Hartnell incarnation, though, it means the true history of the TARDIS is every bit as mysterious as the Doctor herself. Because Martin was traveling in the same TARDIS, with a broken Chameleon Circuit that had trapped it in the shape of a Police Box. Until now, the Chameleon Circuit was believed to have been broken in 1963, when William Hartnell's Doctor settled in the junkyard at Totters Lane. Presumably that's not the case — and, oddly enough, the Doctor should really focus on the mystery of the TARDIS first.
The TARDIS Has Archives Of Past & Future Console Rooms
Back in Doctor Who season 6, the soul of the Doctor's TARDIS was transported into a human shell in an unforgettable episode called "The Doctor's Wife." This episode saw the TARDIS reveal just a few of its secrets to the Doctor — notably, the fact the TARDIS doesn't properly delete all of the old console rooms. "I archive them, for neatness," the TARDIS incarnate explained. "I've got about thirty now." The TARDIS claimed many of these came from the future, but her relationship with time was demonstrably confused; she was bewildered when it came to differentiating between past and future. They could well include pre-Hartnell console rooms.
It's reasonable to assume the Time Lords went to great lengths to hide the truth, erasing records from the TARDIS system as well; that would explain why the TARDIS believed she'd only been traveling with the Doctor for seven hundred years in "The Doctor's Wife." But they're unlikely to have dug quite so deep into the TARDIS systems, especially given that — like the Doctor — they may have had no knowledge at all of these deep archives. If the Doctor wants to learn the truth about Jo Martin's incarnation, and potentially unravel some of the mysteries of Doctor Who's Timeless Child, all she needs to do is access these archives.