I'm a fan of perpetual mystery in fiction. The fate of the Blue Wizards and Ent-wives in Lord of the Rings. The driving force for the return of Honored Matres in Dune. Akavir and the Dwemer in Elder Scrolls. The answer to all of these questions is always going to be less satisfying than the mystery itself, and the presence of these mysteries creates the illusion than the fictional setting is more expansive than it actually is.