You unlock the other playable factions by completing either a short campaign(15 cities & kill another faction) or a long campaign(get 50 cities including Rome). If you really want to play as another non-Roman faction do a short campaign. I haven't played Rome: Total War Unmodded in a really long time but I think the unlockable factions are Seleucid Empire, Greeks, Egypt, Parthia, Carthage, Germania, Britons, and Gaul. There's an all faction mod that you can download that can make ALL factions playable so you might wanna look around for that.
Hope this helps .-Dominicus
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