Well here's my opinion for the four factions:

1-A Belgae faction. I think that this is getting pretty obvious. There's no confirmation yet but with all the hype...

2-A Celtiberian faction, just to make Lusotana nigh-impossible lol

3-I think Scythians may be possible but it may make the place too crowded with BK and Sarmatians, else may be Nabatea; I don't think we need an Ethiopian faction, Saba had already all the regional options and had Ethiopia as a Homeland region, and the culture slots are full...

4-I think they might use this slot for the Yuezhi (Just like the MTW 2 Mongols and Timurids), or they might save it for scripted events, since I think I remember them saying they took out the Seleukid breakup, anabasis, shahanshah etc. because they required a faction slot.