WI Brutus and Cassius win at Phillippi ?
Here's a scenario I envision regarding how that could happen:
Slightly different weather patterns lead to a messenger ship carrying the news of Admiral Ahenobarus' victory over the triumvir fleet in the Adriatic arriving in time to inform Brutus and Cassius.
Knowing that their enemy is trapped and all they have to do is wait, the Liberators army morale skyrockets, whereas Mark Anthony's and Octavians men become ever more desperate. With desertions and low morale plaguing their army, it's the two triumvirs that launch ill-fated attacks against fortified positions and eventually succumb to a counter-attack.
Mark Anthony is killed while Octavian tries to escape. Eventually he is caught up by the superior enemy cavalry at some fortified village. Knowing everything is otherwise lost, Octavian proposes a 1-on-1 duel with Brutus, which Brutus accepts since Octavian is just a sickly teenager and killing him personally would greatly enhance his popularity. The duel's outcome is a forgone conclusion.
I would expect the Liberators to march east, link up with their fleet and then land in Italy. But then what ? How would the Roman world develop, both short and long-term ? Would some sort of reformed republic emerge or would it descend into renewed civil war at the first possible chance and then increasing autocracy ?