I personally want to see:

the scythians to counter the Sarmatians
bastarnae if possible, popluate the NE map a bit more
a Germanic tribe to rival Sweboz, possiby Teutons
a british tribe to counter casse
an Irish tribe (or maybe even 2!) to fight over Hibernia
Another Arbian tribe to rival the Sabeans OR the Nabataean kingdom
The Mauryan Indian Empire. kinda like Parthia in vanila Rome, way at the edge of the map
an celt-Iberian faction to rival Lusitanians
an Ilyrian Faction OR maybe Pergamon
the 2 Numidian Kingdoms
the Belgae

allof these factions would definatley balance out various regions and make EB another 100x more addicting than it already is