Arya's been reciting her 'kill list' - the names of all the characters she wants to kill - ever since Season 2. That's a long time to hold a grudge, and now that she's back in Westeros she could start checking off a few more names.
So far, those to have met the God of Death are: Polliver, Rorge, Joffrey Baratheon, Tywin Lannister, Meryn Trant, and Walder Frey, while the Hound was believed dead, and Arya no longer includes him on her list anyway.
That leaves Cersei Lannister, the Mountain, Ilyn Payne, Melisandre, Beric Dondarrion, and Thoros of Myr. It's unlikely Ilyn will return, even if Wilko Johnson has thankfully recovered from cancer. Beric and Thoros will be fighting Beyond-the-Wall alongside Jon, meaning Arya won't get a chance to kill them, while Melisandre is at Dragonstone, so again should be spared.
That leaves Cersei and the Mountain. It's unclear if the Mountain can even be killed, but if so it seems unlikely Arya - who'll mostly be in the North - will be the one to do it. Geography plays a part for Cersei too, but really it's just much more fitting if Jaime is the one to kill her, and more likely in terms of where their respective stories are headed. Arya may still be able to tick a few names off - they're all at risk - but she won't personally be delivering the fatal blow.