Alexander's Hetaroi
Deity
I doubt that we are going to get any named barbarians other than the six designated clan. That being said I don't see the problem with Atilla being in the game as a "barbarian" leader considering the Huns aren't likely to show up at this point as a full civ.AND they'd have to be very careful not to use any 'Barbarian' name that they might need later for a Civ - what were considered historically 'barbarians' have shown up as Civs too often in Civ games : see Civ V's Huns, Civ VI's Scythians, and many Barbarian-like Leaders - see Harald Hardradda in Civ VI - show up as Leaders for Civs already. I suspect as a result they are either going to find very obscure 'barbarian' Leaders or use completely fictitious ones (Atlatl the Hun, Gonad the Barbarian, Ydvig the Viking) - but not any that might be in copyright or trademark, like ones already in use in popular media.
However the idea of a Hun riding a horse with an atlatl sounds even cooler, yet terrifying, at the same time. I'm thinking of a 4 movement Hul'che unit.