This question is speculative and thus better suited for /r/HistoricalWhatIf.

Thank you.

To start with he probably shouldn't have illegally kept control of his legion in spite of the Senates decree, nor should he have Marched his Army on Rome. The leveraging of the personal loyalty of the soldiers and military force in the city of Rome were the immediate (not systemic) causes of the fall of the Republic.

The best thing Sulla could have done would have been to lose the Social War (or win it less well) and facilitate the creation of a Pan-Italian state and an expanded franchising system included the aristocrats from all over Italy and all the soldiers in the legions. Some sort of new arrangement that kept the power and resources of the state far larger than those at the disposal of its most ambition filled leaders.

A gross over simplification, in the classical Hellenic world, when the power of the state was concentrated into a narrow oligarchy it left itself open to a Popular Revolution and domination under a Tyrant (in the illegal autocrat sense). Which is effectively what happened to the Senate in Rome. Sulla was on the conservative / reactionary side trying to maintain the status quo while the Republic faced a range of external and internal pressures and was never going to be the person that fixed Rome.