After peaking with five successive consulships, the defeat of hundreds of thousands of Germans looming over Italy, and being named "the third founder of Rome", Gaius Marius hit a rut in his older age. Marginalized and uninfluential, his yearning to recapture his military glory led to his contention with his former protege, Sulla, in the command against Mithridates. The resulting civil war led to Sulla's march on Rome and rule as Dictator, which certainly have presented a tempting precedent for Caesar, et al.
What if Marius was cut down in fighting during the Italian uprising of 91 BC, and never had a chance to begin a crotchety old civil war? Could the Republic have lasted a few more decades in the absence of Sulla's edifying breach of power?