This is where I lean as well. The Nine represent the nine sentient beings of the Destiny Universe. They are charged with keeping balance. With the almost destruction of humanity, the balance was disturbed and Xur was sent to assist. I would disagree we have met them all. I count Human, Awoken, Hive, Fallen, Cabal and Vex. I assume we will meet the the Tiger people mentioned in the Festival and add them to the list.