Caesar was also a peacemaker between Pompey and M. Licinius Crassus, when their rising animosity threatened a clash after their joint consulate on 70 BC. Crassus was jealous of the military power and reputation of Pompey, made personal by the incident of Pompey's return to Rome from his conquest of Hispania, when he encountered the remnants of the Sparticists, whom Crassus had been pursuing from the south in his prosecution of the Servile War. Pompey cut down the remaining fugitives and then claimed that he was the one who had ended the war.
Without Caesar's arbitration bringing about a reconciliation between (and eventually the political alliance of the Triumvirate), the civil war might have been sparked twenty years before the Rubicon (maybe by a disputed election or governorship). Pompey would have had the immediate military advantage - "I can stamp the ground and bring three legions forth", but the cautious Senate might rally around the perceptively weaker and therefore more malleable Crassus as a counterweight. Similar to the role that Pompey himself played OTL. Butterflies might accelerate or aid the revolt of Cataline, an advocate of radical reform in favor of the plebs. Maybe a Pomepy-Cataline alliance, with Pompey, the New Man after all, the champion of the People?
http://en.wikipedia.org/wiki/Catiline
One of the characters analysis I've read between Caesar and POmpey purported that Pompey was too conscious of his humble origins to turn his back on the aristocratic system, from which he finally gained recognition and acclaim (similiar to Cicero). Caesar was of the founding families of the Republic, and he was fond of claiming blood lineage back through Aeneas and Venus. This rock-solid security of sangrael gave him the confidence to repudiate the herd-mentality of his fellow aristocrats, desperately trying to keep out new Men and wealth in a jealous clinging to ancient patrician privileges.