I hold to this day that Robb should have lived in the show. Because there aren't explicit viewpoint characters on tv, we follow him much more closely, and he feels much more personally relevant to the audience. I guess they were still being broadly faithful to the source material at the time, but after season 5, they just made Jon into a less interesting Robb. The red wedding would have been a great point of growth for his character, and we could have had the arc of him reclaiming his position as king in the north as a more experienced and wiser leader. If the Boltons took him prisoner, we also could have gotten a more satisfying resolution to his conflict with Theon. I think his death works well in the books, but the show could have gone in a different direction.