Well, I think you only have to remember what they have said all aloong. There would be sequels and add-ons.

I don't know if Warlord will be a true add-on or a new release of Shogun. But they also announced before that new releases would include new settings, so yes, there will also be a new setting for Total War in the future. Probably it will be Romans. And yes, they are already working on it.