I'm talking a total reversal of OTL.
How about Alcibiades scoring some sort of diplomatic power play against his rivals as PoD? Sometime prior to the Sicilian Expedition.
He manages to sidestep Nicias' insistence that a larger force is needed, and the attempt to sandbag him around the time of the expedition's departure doesn't go ahead.
Alcibiades leads a coalition of Athenian and Sicilian forces, captures Syracuse, and perhaps destroys a significant portion of Sparta's allied navy.
Sparta continues to fight against Athenian naval supremacy, no game-changing event in Sparta's favour occurs. This avoids the coup back in Athens, and Athens keeps the initiative in the war.