Arya is a cool character and has been since Season 1, but it's about time she actually contributed something to the overarching story surrounding the Iron Throne. Killing Meryn Trant was the first step because his death will ripple across the Narrow Sea and reach powerful ears in King's Landing, but she's limited on that side of the ocean. It doesn't have to be immediately, but Arya - who is currently blinded - would be more than welcome in any of the Westerosi storylines, though a one that could see her fight on the side of a Northern rebellion would be a great move. It would be even better for her to make a Westeros return with the ability to disguise herself like the Faceless Man.