Lepidus would have to contend with local Iberian and African factions, while attempting to join the two parts of his territories together. Octavian would have to keep control of the north against barbarian incursions, while dealing with the other Roman factions. I could see the Iceni probing and pushing Gaul, which would mostly consist of client states to Octavian’s faction. The remaining playable factions appear to be: So for example, Lepidus might have Iberian swordsmen and Libyan infantry, while Octavian might have access to barbarian infantry, and Mark Antony able to recruit perhaps a mix of barbarian and Greek units. My thoughts are that each Roman faction will feature legionaries as the backbone of the army, but also include regional units as auxiliaries. The four major Roman factions appear to be:

The (Potential) Playable Factions of Imperator Augustus

It features the original grand campaign map for Rome 2, but with different playable factions and starting positions. The Imperator Augustus Campaign Map has been released.

Total war rome 2 campaign map