To playing as a faction without having to complete the game, first train an army that fills a town. Take half the army (and general, in case you need to hire mercenaries) and take out the faction that you want to play as (for example, Egypt, Parthia or Britannia). Go to the menu and it should appear as a faction you can play as. You can now start a campaign as them. Note: This will not work for certain factions, such as the SPQR.