I can see Arya getting back into the main plot in a really GRRM way. He did, after all, say the ending was bittersweet.
My prediction... The Wise Masters hire the Faceless Men to kill Daenerys. She is really the only person in the story worth assassinating at any cost. She holds the Khalsar, the Unsullied and the Dragons together. Without her, it all falls apart, so she is the person who is the highest on the assassination list.
The Faceless Men attempt to kill Dany in Essos but fail. She leaves for Westeros soon after and aligns with Jon Snow, Sansa and Bran in some way. Out of desperation to fill the contract the Faceless Men send Arya to kill her,AS ARYA, because she is the only one that can get close enough to do it and really because of the conditioning, Arya doesnt really exist anymore. Then we get a few dozen chapters of Arya struggling to figure out who and what she is, eventually ending with her killing Dany and then being killed by Jon.