If GRRM were still writing: the waif killed Arya in the dark, gave both herself and Arya a face-ectomy, and the person talking to Jaqen in the final scene is the waif wearing Arya’s face.
This has the GRRM-like quality of killing a beloved character who behaved foolishly and did not have the skills or resources to continue living - Arya should die. And the entire faceless man shifting-identity plotline is no longer a complete waste of time. A girl (looking like Arya) returns to Westeros to murder-genie everyone that Arya hated and restore the cosmic death balance (for some reason).
Since the same actress continues to play FM-Arya, everything looks just the same, so the Arya story does not die. We get a reveal much later that FM-Arya is really the waif.