Ruby Sunday AU (Doctor Who)

If interacting the Doctor him and others: she's the God of Stories and therefore Weird Stories will happen, which then Ruby can fix; not only she'll keep having adventures after leaving the Doctor, but every now and then a part of her that has meta-knowledge will speak to fix an issue and Ruby won't remember it -- until the day she finds out the truth, and even then she can only hear what happened from others, could never remember it herself.
To be clear, she KNOWS something is wrong regardless, the 73 yards episode never fully left her head, and she will assume she must be some horrible thing if she has that effect (the running away forever), especially as more memories from that episodes come back to her. Every now and then she is also taken over by that Ruby, who was even rejected by her mother, and freaks out, but doesn't remember much of it after.
An excuse for her to still be the Doctor's part time companion: since adventures keep happening to her, but at the same time they are not enough, eventually she'll feel compelled to call him for some more, especially because by the end of the season Ruby is finally allowed to have way stronger emotions, including negative ones, which she couldn't have earlier for Plot Purposes. 

Now, about the AU-ness:

When the Doctor brought back superstitions using salt at the end of the universe, he reopened the door for all kinds of gods and weird creatures, so what if the GOD OF STORIES came through and in a timey-wimey way followed the Doctor to Ruby Road, saw a baby abandoned at Christmas with so much potential and realized that that is ITS FUTURE, so it goes back to let it happen and be born as Ruby? Live this new story from beginning to end. 

And so Ruby in "73 Yards" is able to create a bubble universe just to survive, and her real self takes over by appearing to Gwilliams and others to fix this awful timeline, to bring herself and the Doctor back to the OG verse. The fact that it can rewrite timelines/change stories alone could make her all-powerful, except that having that power would take away all stakes and no story would be compelling, so not only human Ruby could never control such power, but the God of Stories isn't interested in using it much.

And that is why when Ruby steps on a prehistoric butterfly and the universe is changed she's still there, because stories can't be erased; why she has a song in her heart to fight Maestro, why she can make snow when sad (it's literally for drama!), why Sutekh feels compelled to find out more about her mystery, because it can feel there is more to her. She's THE story. 

It's a neutral/benign entity and when she dies the Story will just move on (and in a way so will Ruby, who will be reborn as someone else but still be her in a way). Ruby's backstory has played out now that she's got her mother, so Ruby will suddenly be able to have more abandonment issues, feel unsatisfied, and so on. She keeps forgetting things she has said or done to the point that she had to do a few MRI and many tests because worried, but it's all because the God-side has stirred.
Whether you think of IT in its real form or of it in Ruby-form, the attachment to the Doctor became real pretty fast.

One of the ways to see that she's the God of Stories is to be at a distance of 73 yards from her and to really look at her. You are not supposed to see the 'full story', and much like the Boogyman could strike fear even in the Doctor because it was "meant to do it", it will cause immediate fear and a refusal to think about it ('ask her!') as a defense mechanism.
The Doctor can get in control of his emotions like with the Boogyman, it's fine! But people instinctively will not look at Ruby from that specific distance unless Ruby herself (young or old) makes them. 

Also a canon divergence: while I do respect that some adopted kids will keep considering their biological parent 'their real parent', calling Louse 'my real mom' was a bit much, so to my Ruby they are both her mothers, Louise is her bio-mum, while Carla is just 'mum'.
Ruby is struggling with the fact that Louise never looked for her out of fear of Ruby hating her while Ruby was desperate to find her, and then struggles with feeling guilty over not being ecstatic, and that's true regardless of the fears that seem from the episode 73 yards.