I don't know if this is a secret or just my wishful thinking but...I was thinking that maybe the Tau evolution and the sudden appearance of the Ethereal Caste was actually the work of some surviving Old Ones. I think their purpose is to unite the galaxy against the C'Tan (that being the Greater Good). Not only that but the Old Ones learned from their mistakes with the Eldar and the Orks by making this new race barely visible in the warp. The other thing that got me thinking about this is a quote from Eldrad Ulthran in the first Tau Codex. He says that he feels protective of them and that they'll go on to great things. I don't know if it holds water, but it might be an interesting secret.