Last we heard of The Blackfish, he escaped from Riverrun with Robb's widow with him. Where is he going now and will he factor in to any future storylines?
It's fairly obvious he can't really go North...unless he meets up the Howland Reed, because the North is Bolton territory and it doesn't seem likely he has had the opportunity to contact Stark loyalists like Manderly.
However it is possible he may run into Stoneheart and her gang in the Riverlands. That is a meeting I'd love to see, I wonder how he'd react to seeing Cat as a vengeful zombie.
The most likely thing to see in my opinion is him going to the Vale and meeting up with Sansa - this could have a bunch of implications as surely he would know what Sansa would look like, seeing as she is the splitting image of a young Catelyn? Him commanding the Vale armies as they finally enter the war under the clever leadership of Sansa would be brilliant, Maybe he'll even be the one with the guts to kill Littlefinger.