If Michelle Fairley isn't returning for whatever reason, then there are really only three plausible options for who takes on the role of Lady Stoneheart in my opinion. 1) Talisa Stark - Probably the most logical option, as she has the closest ties to both Robb and the Red Wedding than anyone other than Catelyn. Just as Lady Stoneheart does, she has a distinguishing wound that could be as equally definitive of her character as Catelyn's cut throat is. 2) Beric Dondarrion - It may be that he feels some guilt regarding his treatment of Arya. This, in addition to a sense of honour and nobility could mean that he desires vengeance on the Freys for their actions. 3) Brynden Tully - I've read a lot of speculation that he could be the one to take control of the Brotherhood in the WoW. If this is true, then it's perfectly logical that he should begin his ties with them now. He was also present at the RW and so knows exactly who was responsible, whilst his family ties to both Catelyn and Robb make him an ideal bringer of justice to the Freys. Now, obviously these suggestions could create problems regarding Brienne's storyline, as none of them have ever met her (?). Nonetheless, I feel that the character is too important to simply cut, and so if Michelle Fairley doesn't return, then for me it has to be one of these three. For the record, I hope that she does come back as Lady Stoneheart, if only because she's been in it since episode 1 and I'm not ready to say goodbye yet.