In this 'what if' section, we answer the question: What if Spartacus had come to a peaceful agreement with Crassus in 70 BC?
One of the most complicating factors in the Third Servile War was the fact that Rome was already embroiled in two major overseas conflicts which necessitated the absence of the Roman legions: Pompey 'Magnus' was suppressing Sertorius' revolt in Spain, while Lucius Lucullus was continuing the Roman campaign against Mithridates in the East. Not only did this mean that two of Rome's greatest generals were absent but also that a number of Roman legions were engaged elsewhere. Those forces and generals who were sent against the revolting slaves were most definitely the 'B Team'. Crassus, who in reality rejected a settlement offer from Spartacus' team, was not an imperator but a businessman and allegedly the wealthiest man in Rome. A swift resolution of the crisis would have been his priority to avoid the intervention of his younger rival Pompey. The man who saved Rome could certainly expect a political reward and Crassus was nothing if expedient.
A negotiated settlement is unlikely but not impossible: Spartacus had shown himself to be a competent military tactician and had won several victories over the Roman legions. Moreover with Rome's focus on the threat from Mithridates there could be a case for a settlement until greater resources were available. If Mithridates was the bogeyman used by Roman mothers to scare their children, Spartacus was the one used to frighten really naughty children.
So the ramifications of any settlement would be twofold: political and social. While Crassus defeated Spartacus in battle, Pompey mopped up the rebellion, enabling him to claim credit as well. They both stood for the consulship the next year in a temporary political alliance as a result of their prestige. A settlement would have prevented this alliance so perhaps aided those adherents of the Pompeian cause, such as Cicero. However, if Pompey had returned from Spain and reneged on a Crassan settlement, defeating the rebels on his way home, his prestige would have gone through the roof. So the widest political implications are that Pompey becomes the leading figure in Roman politics at the expense of Crassus and perhaps even Julius Caesar. Yet while the cause of the extreme populares might have been set back, this perhaps would have furthered the intrigue of those such as Catalina whose revolution was later thwarted by Cicero. However the fact that Crassus would be a discredited figure would perhaps have ensured that the triumvirate never happened and that Pompey remained firmly on the moderate senatorial wing of politics (like Cicero).
In actuality, the Third Servile War (the first on Italian soil) lead to Roman citizens treating their slaves less harshly. A settlement, especially if Pompey did not intervene, would have furthered this: perhaps some legislation (as was later done by the Emperor Claudius over a century later) would have been enacted to lesson the harsh conditions in which slaves existed and landowners would have increased their reliance on dispossessed landowners rather than slave labour from conquests abroad. So perhaps the era of expansion in the late Republic would not have happened. Perhaps Caesar would not have been given his extraordinary command in Gaul (which he only got due to the triumvirate). That, however, would be to underestimate the cunning of Caesar...