Most of the world is divided into sovereign nation-states of some sort, usually based around a common language and ethnic identity. There are some federations like the USA and India, and some unstable or failed states, but geopolitics in Europe and west Asia is dominated by nation-states...
Well, that implies that stopping the collapse of the classical states is the easiest way to keep the Roman-type imperial nations around. A lot of timelines successfully do this. Still it's harder to merge nations than split them, and every attempt at a pan-whatever superstate or reviving the...
If you look at a map of the Roman Empire, you can see it encircling the Mediterranean. Even the Byzantines and the Ottomans seemed to bend around the Mediterranean. Or if you look at the Phoenican thalassocracy or the Delian League, you see loose networks of cities, yet it's still a very fluid...