Here is the link to Michelle Fairley's imdb page
http://www.imdb.com/name/nm0265610/
>On Game of Thrones from 2011-2017
> Latest Episode: Season 7 Episode 1
> Character: Mysterious Lady
Make of that what you will.
Could possibly be a flashback, but hopefully we will see Stoneheart.
But it seems a bit unlikely with only 14 episodes left, not much time to develop it.
I think that we won't see Stoneheart, why? Because as we saw well, Arya started extermination of House Frey, so D&D needn't her, btw with whom she will be work? Dondarrion and his party are going to the wall.
Stoneheart wasn't even developed in the books that much. It's too late to bring her in now. The time for that was two seasons ago.
I'm not sure how she can advance the show anyway.
Towards the end they should be killing off more characters than they create.