In the Imperial Campaign, by default, when you first start you can only play one of the three roman families. Any unlockable faction you personally eliminate from the game becomes playable from that point on, and winning a campaign - even a short campaign - allows you to play all the unlockable factions. Again, by default, the Gauls, Britons, Germans, Dacians, Scythians, Greek City States, Seleucids, Parthia, Egypt and Carthage are playable. Armenia, Numidia, Spain, and Macedonia are not. I can't remember if Thrace and Pontus are playable or not. There's a file that lists them in individual sections: one is unlocked, one is unlockable, and one is unplayable. You can move the names into the "unlocked" section to make them playable.

NOTE WELL: do NOT move SPQR into the playable section. The game crashes if you try to play the senate.