The likely answer comes from way back in the show's first season, when Arya created a kill list of her enemies — and she's made a pretty sizable dent. Notable members include Ser Meryn Trant (dead), Tywin Lannister (dead, courtesy of Tyrion Lannister), Joffrey Baratheon (dead, courtesy of Olenna Tyrell), Walder Frey (dead), The Hound (now her friend), Melisandre (dead, but redeemed), The Mountain (alive, but as a zombie), and Cersei (alive, still terrible). Realistically, the latter two are the only characters on the list Arya has left to defeat, and with the Night King behind her, it makes sense that both she and the showrunners would want her to round out her storyline and tick those boxes. So, could be Arya going to Kings Landing with the Hound to kill the Mountain, and then Cersei (who happens to have green eyes, along with Daenerys) . But what would she do after that?