Could they do Viera, sure. Iconic? Hardly. It was featured strongly only in the lest popular numbered game in the series, but already noticed how double standards are applied in this argument. Forcing any race from another game into this game is as invalid as asking for a brand new one, either way, they would make zero lore sense without retconing them into the universe. Anyone that actually cared for lore of the game, would seek among existing beast tribes.