This is slightly off-topic, I realize, but if Vercingetorix's plan succeeded, Caesar was killed, and the Roman Legions were expelled from northern Gaulish territory, the Arverni-led alliance might dissolve, but everyone will remember that it was under the leadership of that particular tribe which freed the Gauls in the first place. This may have done much to boost the nationalistic self-esteem of the Arverni. Neighbouring smaller tribal bodies could have aligned themselves to the Arverni for protection and paid tribute to them.
Discharged Gallic auxilliaries from the Roman Army might return to Gaul and hire themselves out to those that require warriors to build up a new standing army. Vercingetorix, having served at one time in the Roman Army, was said to have trained his fellow Arverni in the Roman mode of combat. Perhaps the surviving Vercingetorix would use his newfound prestige to build up a new model army from all this.
The Aedui tribe, being divided between the Pro-Roman faction (one notable member being the Druid Diviciacus), and the Anti-Roman faction (once led by Diviciacus' brother Dumnorix until his death in 54 BCE). A purging of the aristocracy among the Aedui could be the beginnings of a new civil war that could sunder the tribal confederation. Vercingetorix or his heirs could use this as an opportunity to expand their control over one of their nations oldest enemies.
Having established proper control over all of central Gaul within ten or twenty years after Caesar's failed conquest of Gaul, the Arverni could lead new expeditions to subdue the Belgae alliance in the north, the Aquitanii in the south, and swallow up the tribes in the regions of Brittany and Normandy.
By the end of the First Century BCE, with much of Gaul united by the military might of the Arverni, and with an abundance of precious metals and iron deposits to help pay for and maintain their new professional army, the Gauls could next wage a military campaign to capture the Roman province of Narbonensis. Thus gaining access to the Mediterranean sea.