The existing one has the first series/season (10 episodes) begin with Caesar in Gaul, covers his war against the Optimates, Pompey and his followers, and ends with his assassination.
Keep that. the second series/season had the problem that it was two series/season's worth of events in one. So:
The second series/season is about the war of the Second Triumvirate (Octavian, Antonius, and Lepidus) against Brutus and Crassus, ending with Second Philippi, where Brutus gives up and is killed.
The third series/season can deal with the wars between Octavian and Antony & Cleopatra. We can have a lot more of Antony's blunders, including his less-than-successful campaign against Parthia, and the wild pageant where he declared Caesarion and his children by Cleopatra as kings of various parts of the East. Same ending, but a lot more background.