After finishing your first short or full campaign,some more factions will be playable.As I can remember,they are Egypts,greek,parthian,seleucid,german,gaul,briton,carthage.You can also make others playable by modding the file descr_strat in your game directory.
Barocca,I dont think defeat a faction you can play that faction.I am able to choose Egyptian ,but I never defeat them before.