Remember Arya's final bout with the Waif back in season six? While it's clear the women fight to the death, we don't see anything about their battle once the candle goes out. We assume Arya prevails because we later find her confronting Jaqen H'ghar about his role in the ambush — but what if Arya actually lost that fight?
Wired suggests the Waif killed Arya and has been wearing her face ever since. The argument hinges on Jaqen's parting decision to bestow Arya with the title of "no one" despite his supposed anger over her missteps, such as murdering Meryn Trant without his permission and her failure to complete the assassination of Lady Crane. Not only is this sudden change of heart out of character for Jaqen, but the term is one he usually reserves for the Waif. Thus, we could interpret his farewell as an indication that he recognises the face behind the mask.
Granted, one could easily discount this theory by saying Arya has been diligent about completing her kill list and that the Waif would have no reason to follow the young Stark's personal vendetta. However, the Waif is an assassin of the highest order, so wouldn't she have just as much fun killing for sport as Arya does for vengeance? So, here's the real question: if the Waif lives, what's her endgame? Clearly, death to Daenerys Targaryen. Yup, you read that right. Remember, the Lannisters once considered using Faceless Men to kill Daenerys prior to her coming into power. Maybe they never fully gave up on that plan and we're heading for one hell of a plot twist where Arya appears to murder the Mother of Dragons.