Robb, or Joffrey? Those are the two real options, right? From a story telling standpoint, I'd guess Robb is the more likely choice, as you could do a little more of that up front, and not really miss it later. It does make things go to shit a lot faster for the Starks, though, as Caitlin run into a little trouble of her own there...
Joffrey has more potential as a story you could work with longer, although I guess you can just play into the Tommen story earlier?
Only other guess I've got could be the Lord Commander of the Night Watch, promoting Jon Snow earlier than intended. That would condense a lot of his story, but if you gave a little more for him earlier, it would solidify the situation in the North earlier than in the novels...