The Great DividePerhaps a small group breaks off from the herd? Either through a disagreement or a threat/disaster of some kind.
The Unknown Line (later later on)This is a backplot of sorts. OCC Xanti's second breeding is with Natty's Vera. IC this breeding actually took place before Xanti met Drith and the herd formed. This part of the plot is basically a reunion with one (or perhaps more) of the offspring. Pretty much Chaos ensues.
Other ideas?Post in thread if you have more plotting ideas!!