Check \EB\Data\world\maps\campaign\imperial_campaign\descr_strat.txt for playable and non playable factions.
The default looks like this:
Spoiler Alert, click show to read: 
Code:
playable
	seleucid
	pontus
	scythia
	parthia
	numidia
	romans_julii
	egypt
	gauls
	germans
	britons
	greek_cities
	macedon
	carthage
	romans_scipii
	dacia
	armenia
	spain
	thrace
	romans_brutii
	saba
	slave
end
unlockable
end
nonplayable


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