Rebel forces - yes, playable faction - no. Slavs tribes were known in Roman world as Veneds. Some historians think that ancestors of slavs appeared in Northern Europe in times of Herodotus but lack of evidences made those versions rather unproved. Slavs did not play any significant role in ancient world, so including Slavs as playable faction would be historically inaccurate.
BTW Hungarians and Huns are two absolutely different nations. Hunns are supposed to be from Chinese area (hunni, chinese tribe, lived to the west-north of Beijing). Hunn movement from Asia to Europe was accompannied with accumulation of a number of tribes of asian and european origin (incl. slavs) in hunn's army.