From the North, off the top of my head, you have Umber, Karstark, Mormont, Forrester, Manderly, Glover, Hornwood, Cerwyn, Cassel, and Reed. Is the Reed house big? Don't forget the secret that House Reed, or one specific member, knows. I don't know how big any of those houses are, but just because they haven't been mentioned thoroughly doesn't mean they won't play a part. Obviously we know certain ones, such as Mormont, are irrelevant, but I would imagine there are others we haven't considered.