CK2 and 3 already have a few drafts scribbled out, so expect to see them at some point in the future. However, my next game after CK1 won't be a CK game (though it will be set in the same universe). After three-plus years, I need a little break from these characters ;)
There may also be a few spin-off CK games that I release as a way to practice and refine new mechanics/engines outside of a mainline game.