Great assignment! This is a very open question, so in a sense, how you respond will depend a lot on you and what stood out to you as being the possible basis for a sequel for this brilliant story. I will give you my ideas, but please do go with your own ideas and just use mine as an example of the kind of thing you could write about.
To me, there is one mystery in this story that is never fully explained and which would make the basis of a brilliant sequel. This is of course the reasons for the narrator's return to live with her mother. Note what we are told about this in the story:
Since my father's recent death, there is no one to read to her, which is why I returned, in fact, from my failed life where the land is flat. I came home to read to my mother, to read out loud, to read long into the dark if I must, to read all night.
Clearly, there is more to the "failed life" that the narrator is running from. As the focus of the story is how the mother saved her daughter twice, a sequel could focus on how the mother is able to save her daughter a third time through helping her to deal with whatever constitutes her "failed life" and moving her on into wholeness and healing again.