One of the main reasons for not having a 3rd expansion is that they ran out of $$$ patching Entrenchment and making some major changes that they never anticipated. The end result was to cut out the 3rd expansion which only hinted of a 4th race (find the April fools joke about the Care Bear race ).
For now they are pushing the envelope to wrap up diplomacy, waiting for sales on the game, and finishing up with balance issues. The rest will be left up to onliners, modders, and anyone at Ironclad keeping tabs on us.
Put it this way, I've waited a long time for a game like this, and it's one that I'll be playing for even longer.