In an earlier thread, something like this came up, with the POD being that Caesar was defeated by Pompey during their civil war. Pompey and the Traditionalists' victory means that Caesar and Antony both kill themselves, and so end that round of civil war. The Republic continues shakily along.
Agrippa has the military skill and political connections to end commanding the successful invasion of Parthia, and the wealth accumulated from that conquest allows him to prosecute a successful round of civil wars and end the Republic.
Octavian could probably survive his great-uncle's death, and he has a LOT of political skills, so he probably makes a later emergence. Rather than being the leading figure in the end of the Republic as he was OTL he could lend some gravitas to Agrippa's entourage. In this TL Agrippa's position as a 'new man' might not mean as much, after all, Pompey saved the Republic, and he was a 'new man' too. So Octavian ends up in a similar position to what Agrippa was to him, only as a kind of prime minister, taking care of messy Roman politics while Agrippa thinks deep thoughts and wins great victories.