Act Three will be the final act, but it will be followed by an epilogue that ties up most of the loose ends, allows you to explore and see the changes you wrought, and sets up the sequel.
As for the future, I have two full sequels planned - CK2 and CK3 - which are currently in the rough draft phase. No details on those for now.
Also, a number of players have put forward ideas for spinoff games, which... who knows? Maybe one day :)