Aw. That's too bad.
I personally liked the idea of Cat coming back because I liked her character a lot, and even though she's not Cat anymore, she still kills a lot of Freys. I won't go too into detail, but I feel that Stoneheart will become very big in TWOW due to how ADWD ended. Of course, I could be completely wrong. I just feel that if Martin, who is basically god of death, revived a specific character, he'd have a very very good reason. Or I'm just a Cat fanboy.
To CyberEvil (stop reading Brute.), like you said, there's still too little content to know for sure how Lady Stoneheart will impact the story.
I'm really kind of just blabbing here about what could happen with Stoneheart in TWOW. Stoneheart has Brienne's life wrapped around her finger. Since Brienne chose "Sword", it means she's bringing Jaime back to LSH and she chose to lie to him at the end of ADWD. So, there's not much material to go on, but Brienne's honor will not let her leave Pod and Hyles behind, which means she either needs to give up Jaime or come up with a plan. Stoneheart could potentially kill off two main characters + their two friends. I assume that either Jaime or Brienne or both will die in TWOW (I can't really see how they could both escape, but please feel free to argue with me on that one). If Brienne takes Jaime to Stoneheart, she'll either immediately hang/torture him or what I think is highly likely, give him a trial by combat. It'd fit in perfectly into Jaime's arc with his left-handed sword training. It'd be cruel and ironic because Stoneheart knows he can't win. That, or Brienne volunteers to fight for him. Or Stoneheart could make Brienne vs. Jaime.
Of course, there could always be a Deux Ex Machina like the Blackfish.