In OTL, southern Italy (often paired with Sicily) was a united kingdom for 731 years (1130-1861) with 1, clear urban center standing above the rest, Naples. (A historical atlas of mine shows Naples as larger than any of the northern Italian cities, and larger than most northern European cities, through the 1500s).
Meanwhile, central Italy was occupied by the Papal States, and northern Italy was divided into multiple independent city states or regional domains.
The challenge is to reverse this:
With any PoD after 650 CE (after all the Lombard victories against the Byzantines), have southern Italy be split among multiple city-states (some Republican) and other small domains for centuries on end (at least 731 years).
So, for example, Amalfi, Pescara, Trani, Bari, Brindisi, Taranto, Crotoni, Reggio, Messina (in Sicily), Siracusa (in Sicily), Palermo (in Sicily), Tropea, Salerno, Naples, Gaeta, Benevento and Melfi [or substitute some other southern Italian towns/cities of your liking], are not under a common monarchy but instead are separate, and competing, Republics, Dukedoms and Dictatorships with proudly distinctive civic identities and traditions.
---At the same time, with any PoD after 650 CE, get all of the northern Italy, north of Rome (and other Papal States) to be united under a single kingdom for at least a 731 year run. So, northern Italy has a single capital, (for example, Bologna, Florence, Pisa, Livorno, Leghorn, Pavia, Turin, Milan, Venice, Verona, Ravenna - I do not care which), and all the other famous cities of the region are simply subordinate provincial towns.
How's this for a PoD:
Could we get something started along these lines by having Norman adventurers seize and consolidate a state encompassing Italy north of the Papal states, breaking it from the HRE, but having Norman "northern Italy" be hemmed in from further expansion southward by its own overextension, Papal resistance and the Norman northern Italian state having to fight for its life against HRE attempts to reconquer it?
Meanwhile, south of the Papal States, without an enterprising Norman conquest, southern Italy remains divided into small duchies and city-states like Amalfi. Maybe Sicily remains Arab-ruled longer without the Normans, or if certain southern cities like Amalfi, Naples, Taranto start gaining naval/commercial power like the Venetians, Pisans and Genoans of OTL, they can recover Sicily piecemeal, establishing their own colonies on the island?