I believe you have to buy the base game to for expansion. After all, they are selling "A Realm Reborn" + "Heavensward" duo pack at the release of expansion. Though it has been noted you can still play the game with out expansion. You just wont be able to do anything post 2.55 Main Scenerio wise, or experience what the expansion has to offer (things like New Race, Jobs and Ishgard access, etc)