In reading accounts of the Battle of Philippi, it is pretty clear that it was touch-and-go for awhile and that victory could have gone to either side. Suppose that the outcome had been reversed, and that the republican forces of Brutus and Cassius had triumphed over the triumvir forces of Antony and Octavian? Assume also that Antony and Octavian are either killed or (as Brutus and Cassius did IOTL) commit suicide?
What then?
Lepidus is still in control of Italy with a few triumvir legions, and the governors of the Western provinces are men who were loyal to Antony. But the east is firmly under republican control, and Sextus Pompey and his fleet pretty much control the seas. Indeed, the triumvir fleet had suffered a heavy defeat in the Adriatic right before the Battle(s) of Philippi had begun. If the main triumvir army has been defeated and the republican forces easily able to transport their men to Italy, it seems to me that Lepidus wouldn't have lasted long.
The Senate (what was left of it) was pretty much on the fence and I think would have swung back quickly to Brutus and Cassius the moment they arrived in Italy with a few legions.