It's still very possible that Stoneheart will appear. I notice there have been no outright denials that she might, so it could still be in the cards. If so, it would probably happen towards the end of Season 5 at the earliest (and most opportune storywise). It'd be about the perfect time since Jaime and Brienne would likely be done, or nearly done with their arcs in Dorne and The Vale respectively, and if the showrunners ever plan on having them cross paths again then introducing Stoneheart is the way to do it.
I also think it'd be a huge missed opportunity on their part not to bring Stoneheart in, really. It'd be a shock for the ages to both those who've read the books and those who haven't and would honestly work much better for Jaime's and Brienne's storylines than just making up more things for them to do.
So... I really hadn't thought about it that in-depth before, but I'd say there's definitely still a decent chance Stoneheart will appear.