Of all the character arcs on Game of Thrones, Arya's has been one of the most consistent. From the moment she bested her brother Bran at archery in episode one, Arya has been outspoken about wanting to be a fighter, rather than a lady. This was always her goal and, across eight seasons, she has learnt everything she needs to know.
Though Jon may seem like the obvious choice in the Who Will Kill Mad Daenerys? conundrum, he's far from the only option. And if anything, he's a far less brutal, or vengeful, character than Arya. And the programme-makers made explicit effort to show how affected she was by the sight of the burned bodies of the Kings Landing inhabitants, and how she tried to help them.
Arya began training with SyrioForel, a master sword-fighter and former First Sword of Braavos, when she was only 11. By season two, she had memorised a kill list of people who had wronged her. By season four she had murdered her first person from it. The list has rarely been far from her mind (though, as we have seen with the Hound, she is willing to forgive people if they earn it).
She spent season five and most of season six in Braavos learning to be a Faceless assassin. Between losing her sight and spending her time begging on the street, washing dead bodies and committing yet more murders – including an extracurricular return to her kill list to bump off Syrio's killer Meryn Trant – over the past decade she has undergone the most intense period of training and development of anyone in all of film and TV.
It seems a shame – even strange – that we haven't seen her use her face-changing ability since she infiltrated the Twins (the seat of House Frey) and killed Walder and the remaining male members of the family at the beginning of season seven. Surely that cannot be the end of it: all that effort, all that training only for Arya to stop using her best weapon even when faced with a new mad Targaryen on the Iron Throne. But for what other reason would she need to use it?
It wouldn't even be hard to do. Faceless assassins can only take the face of dead people. By wearing either Jorah or Missandei's face, Arya would buy herself enough time for a quick flick of Needle. She could even wear Khal Drogo's face – actor Jason Momoa posted pictures on social media of himself hanging out with the Game of Thrones cast while they were filming the final series in Belfast in 2017. True, it is unlikely that Arya is the "prince who was promised" – but that prophecy is unravelling fast anyway.
Besides, Arya has two potential heirs to the Iron Throne that she's particularly attached to. She may have refused Gendry Baratheon's proposal but that doesn't mean that she doesn't care about the man. He doesn't seem to much want to be king either, but with Daenerys out of the way it could be left to the two former bastards to fight over who wants it less. With any luck, they'd then just let Sansa have the job.