Lady Stoneheart Scene
I'd like to see a scene where Catelyn appears to either Arya or Littlefinger in a dream. First appearing normal then as Stoneheart. Kind of like a nightmarish vision portending death. I could see it work with either Arya or Peter. Either to Arya as an omen that she must kill Peter or to Peter prior to being executed. Sort of like the Ra's Al Ghul scene in Dark Knight Rises.
It'd be a good way to appease those who wanted the character in and a benign scene to those who hated the character.