I've recently started to get interested in Roman history. I've only just started my reading, so I'm sure I've got a bunch of stuff wrong, but fortunately we have plenty of people here who can correct me.
The impression I get is that, unlike the fall of the Roman Empire, there really isn't a whole lot of disagreement about why the Roman Republic fell. The Republic fell because the traditional forms of society - not just the government - no longer really worked. The traditional economic basis for Roman citizens, the smallholder farm, was obsolete; the governmental structure was too unwieldy to handle a massive empire; and the professionalization of the army meant that the soldiers increasingly felt more loyalty to their commander than to the senate. We could add more items to the list, and we can argue about which particular factors were more important, but basically the system as it was could not continue, and so it didn't.
But, granting that the Republic as it was could not continue, did it have to become an Empire? Could the Republic have evolved in some other way? I know that the "democracy" of the Republic was not particularly democratic, at least by modern standards, but could some kind of democracy, or even just oligarchy, have emerged instead of the Augustan dictatorship?