What would you have to do to prevent the spiral of corruption and dictatorship that destroyed the Roman Republic OTL.\? What reforms could possibly be made?
Save M. Livius Drusus from being knifed. You're not going to free the republic of political turmoil- but his reforms would have at least laid the foundations of a constitutional settlement, whether immediately or as something to be turned to later.
But you also need someway of reconciling the need for a professional army with politically minded generals- that means passing some kind of program which guarantees rewards for soldiers will be distributed by the Senate, not their commanding officer.
The fox speaks true. In the span of thirty years Rome went from a city-state barely dominating Latium to a city ruling half of Italy, then all of Italy, then half the Mediterranean, then all the Mediterranean.... Its expansion occurred way too quickly for society and the economic system already in place to adjust themselves or to be adjusted. As late as 100 BC (with Rome owning Spain, provincial Gaul, Italy, Dalmatia, Greece, Macedonia, the western coast of Asia Minor, and North Africa), there were still armies that campaigned for years composed of farm-owning citizens. With them away, those farms went into disrepair and the soldiers' wives and sons reduced to beggary in the cities.
Perhaps a POD of ultraconservatives (like Cincinnatus) coming into power (instead of progressives such as Scipio Africanus was) and declining to rule the world is in order. But then, of course, you have unprotected Roman commercial interests abroad; there can't be tribute kings and chiefs and ethnarchs, either, because a smaller Rome doesn't have the military power to project itself so far. Since Rome's economy depended greatly upon trade, and Roman traders can't succeed in faraway lands without protection (pirates are serious business), and Senators can't succeed unless they support the traders (equestrians/Second Class), this POD might actually be impossible.
There can't be a smaller Republic, therefore the Republic is doomed due to its size. Thoughts?