Badger, first of all, nice work. And secondly, the Riverlands still have a fight in them. After the fall of the Tristifer IV and V, a few hundred years later a new house united the Riverlands under one banner, House Teague was then the King of Rivers. This was before the complete fall of the Riverlands to House Hoare and the Stormkings.
Maybe some more backstory, or an emergent faction? I realize its more work and everything, and this could be before or after the rise of the Teagues, but if you do decide to put them in, it would require a bit of story changing, which I doubt you'll do
But if you do go along with it, a tiny kingdom with some mighty starting armies, such as a riverland rebellion, would be a good way to nudge it in, perhaps high unrest in the regions from Oldstones to Darry, with one revolting heralding a the new Kings of the Rivers and Hills?