Though her stunning exit in Cersei's dungeons offered the chance of an open end for Ellaria Sand, it seems the character is best considered dead. At least according to Indira Varma, the actress behind the character.
She has spoken to EW to confirm that the head of the sadly misused Sand Snakes will not be returning, despite being left alive to watch her daughter die and rot in their prison cell. It seems Cersei's promise that the rest of her life would eke out staring at her daughter's corpse will absolutely be delivered upon:
“[Showrunners David Benioff and Dan Weiss] called me, but I kind of knew already. Obviously there’s lots of trimming going on. It’s all coming to a head and you have to get rid of less important characters that the audience hasn’t had the chance to invest in as much. So I was expecting it. I wasn’t heartbroken. And I was like, “As long as I die on screen…” and they were like “Yeah!” But of course I don’t die on screen. I stay alive, I’m just not going to reappear. I think it’s really clever.”
Not showing her die was an absolute masterstroke, as the idea of her trapped down there with Tyene is so much more monstrous when it plays out in your imagination than anything they could do on screen to Ellaria. It is unthinkable, which is precisely why it works so well.
As Varma herself admits, it is a far more poignant way to go (and it's not like she didn't deserve it):
“It’s really dark. What I love about this scene is you’re reading it and from one sentence to the next you don’t know what’s going to happen — how Cersei is going to treat her victim. I just thought the delivery of that information was so clever. Especially since the kiss comes before the information.”
So those of you theorising that there might be another sting in her tail with Cersei might as well give up the ghost now. Ellaria Sand is no more, and that's not a bad thing at all.