There was actually an essay on this very subject in the first "What If...?" book. Two possible PODs were proposed:
1. A Roman victory over the Visigoths at Adrianople in 378, which buys the Roman Empire enough time to regroup its defenses and reignite confidence and political will. This saves the Roman Empire from its decline and keeps Europe unified under Rome. The end result is that the Roman Empire continues, but what this means is continued centralization and hierarchy, with the pope still subordinate to the emperor, and most likely without any eventual calls for reformation, parliamentarianism, democracy, and the like getting very far.
or 2. A Muslim victory over the Franks at Poitiers around 732, which sees a complete Islamic conquest of Europe, which remains under a unified caliphate. Such a society would also have been hierarchical and slaveholding, but would've enjoyed all the benefits of the Islamic Golden Age, from infrastructural development to the patronage of arts, science, and philosophy to cultural diversity to prosperous trade economies.
In both cases, the author argues that while this would've meant stability and cultural development, it would've prevented the ideas of secularism and individualism that were made possible in the decentralized feudal age, which in turn means no development of democracy or republicanism as we know them.