Good question. I don't think the Mercy chapter we saw is a chapter that takes place early in the book. On top of that, I'm not even sure that the Mercy chapter will even be incorporated into the actual book. Who knows? Maybe killing Raff the Sweetling was an assignment. I think the Faceless Men will task Arya to kill someone in Westeros...one of the Others or maybe even the Night King (if there is one). Why? Arya is very special: females are not commonly allowed to join the Faceless Me