dw sp-oilers under the cut in the sense that me going 'this is how I'd like to write an au Ruby' will inevitably tell you that what I'm writing is NOT what happens and that's still spoilers:
When the Doctor brought back superstition using salt at the end of the universe, he kinda reopened the door for all kinds of gods and weird creatures to show up, and goblings looking for coincidences, and some of his next episodes worked like fairy-tales or tvshows (the boogieman? breaking into songs and almost breaking the fourth wall? a popular theory was that he was stuck in someone's tv)
so what if the GOD OF STORIES was looking for a way to manifest (has always existed and always will) (River telling the Doctor 'aren't we all?' about fairy tales) and in a timey-wimey way followed the Doctor to Ruby Road, found a baby abandoned at Christmas with so much potential, and with an incredibly dramatic mother dressed in a way that would make impossible to see her face, and decided to live through her? A coincidence as well, to find someone so fairy-taley, which attracts Goblins which attract the Doctor, sort of like the Bad Wolf made itself, or the TARDIS brought the Doctor to Messaline because of Jenny, making her existence possible. And unlike the others, she's not evil or seeking destruction, only entertainment.
And so Ruby, who wasn't exactly super 'explored' as a person besides wanting to know who her bio mother is, no particular flaws, in a way makes stories happen. Not always, she didn't cause what happens in Boom, but since her mother can't even be found by the ambulance, she causes a parallel universe to happen with a prime minister who will force everyone to do a DNA test and be in a list, and she does that using the stories she knows about fairy circles and creating that alternate reality. And that alone, creating a time-looped parallel universe with no Doctor, could make her all-powerful, except... she's not interested in that, not as Ruby and surely not as the God of Stories, because once you have a power who can do ANYTHING at will, well, the story is over, no more stakes, so she can't just choose to create a parallel universe consciously, it has to happen to her.
And that is why when she steps on a butterfly and the universe is changed she's still there, stories can't be erased, why she has a song in her heart to fight Maestro, why she can make snow (dramatic!) even if her mother and father are human, why Sutekh feels compelled to find out more about her mystery. She's THE story.
It's benign and when she dies the Stories will just move on, but she's allowed to be more than the perfect companion who is brave and kind (and let's be real, waaay too ready to be ditched in favor of Rogue, she doesn't feel important even if her adoptive mother is wonderful) and since she's not done, she'll DRAW THE TARDIS BACK TO HER. And now that she started remembering things from 73 yards, well, that's a new story and the Doctor can 100% find out that yes, she's human, and also the story, until the story feels complete (which is not now, when 73 years didn't come up, nor why she can make snow, or change the Doctor's memory, and memory is time and all, and why so many coincidences happened around her)