A lot of people feel we need a Dark Side threat, but the Sith need to remain destroyed because of the prophecy and Anakin's sacrifice. Its highly unlikely that they will use an EU character so that is out of the picture. But I think we have seen a new character that fits the criteria: The Son from the Mortis arc from The Clone Wars. Not a Sith but a powerful Force User aligned with the Dark Side. Its my pet theory that the Son survived Anakin's Lightsaber strike and is actually impersonating Darth Maul.