I fail to see why new races are such a problem. They don’t need to be incorporated into the old personal story, at all. In fact, it would be far better to take the opportunity of doing an expansion to create new, simpler personal stories (the branching really isn’t that important and just becomes too much to manage) focused on new playable races in new parts of the world.
In fact, doing that would be a major priority for me if I was in charge of design on this game.