There is a dialog tree, a good one worth 80$ in store. Hovewer I always like using tools that are officially supported. If you add one, It will be much easier to add some rpg element into Unity games. Instead of hack/slash we can do final fantasies(exaggerating) more easily.
That's not something which should be officially implemented in Unity, because it's far too game-specific. It should be a third-party package (with source code so you can alter it as needed), or you can make one yourself. --Eric