I remember someone online making the case that the Spartacus Revolt spurred the Romans to be less brutal to their slaves.
Maybe a worse Spartacid Revolt--perhaps he gets to Sicily and temporarily establishes a Sicilian republic that takes a long time to destroy or he defeats Crassus and or Pompey in the field--leads to the Romans viewing the slavery/latifundia system as a national security threat?
Ending slavery entirely might not be doable, but maybe earlier reforms (such as Caesar's requirement that 1/3 of the jobs be filled by free people) or a ban on importing more slaves could be implemented.