Just a story I was thinking about. I'm not a writer so that is why I am putting up a challenge.
Anyway here is the basics of this plot.
Ruby Rose gets sent to the AOT world. Why or how is up to you. She looses her memories (optional) and meets Grisha. They get married and then they have Eren. When the events of the first episode take place, Erens mom (i.e. Ruby) is pinned under the fallen house. Just then her memories return and, while Eren, Mikasa and Armin are under the impression that she is dead, she survives and then hides from them (or any other reason). At the beginning of the the Trost arc she returns. From there you can go your own direction.
You can change a few things if you want to but that's the basic outline of the story.
So good luck!!!