According to some folks over on Reddit who are also members of the Xbox One Preview Program, we may be getting Cortana. Not the actual Cortana from Halo 5: Guardians, but rather Microsoft's Personal Digital Assistant.
Cortana is already available on devices that feature Windows 10 and most Windows 8 devices. If you're interested in getting a preview of the Cortana feature then you are going to have to sign up to the Xbox One Preview Program.
Thanks, XboxOneUK.