I don’t wanna sound like a Negative Narvin™️ but almost every theory I’ve seen about Ruby has her ending up being the daughter of either the Doctor or a companion of New Who significance (River, Rose, etc) and the absolute dread that fills in me at the thought of that being the twist is almost incalculable. The thought of it is just so boring and so uninteresting to me that if the final twist is something along those lines I will be so deflated.
Personally, I don’t have many theories yet but I stick with my original one and that is that Ruby is what I will call a Paradox Baby™️. This means that the woman dropping her off at the church, her “mother”, is actually herself and we never truly learn the origin of Ruby Sunday.
She doesn't trigger any sort of paradoxical effects by holding her infant self because she was always the one holding her infant self. The reason snow appears when they discuss Ruby’s origin is because reality can’t handle looking too closely at the paradox that is her existence. The Maestro was so unsettled by the (Christmas) song in Ruby because the song is representative of her paradox. It’s why the TARDIS makes strange noises around her every once in a while. Hell, it could be why the whole butterfly thing happened (regardless of what the Doctor said). She is able to exist because after Flux and everything with Swarm and Azure, time and space are nowhere as synchronous as they once were.
Finally, it all lines up with her line from the trailer, “It’s taken me all this time to realize what I’m meant to do. I’m going to save the world.”
Realizing that she has no past to speak of beyond that Christmas on Ruby Road, she needs to look forward to her future, her purpose.
And in a way this all sort of parallels the Doctor’s story with his past. He’s an “adopted” child with no idea of where he’s really from, but that just means all he can do is look forward to the future and what he’s going to do with it.
Hell, not to get too conspiracy theory, but it would also explain the music choice in the primary trailer. Changes by David Bowie is about how you can’t change time but time changes you.
That’s my working theory, at least.











