I see a few chances.
1) Following the unification of Italy, Josè Borjes, backed by the Bourbons in exile and the Papacy, manages to somehow come to an agreement with brigand leader Carmine Crocco. The bulk of the brigand bands of the former Kingdom of Two Sicilies are unified in a semi-regular army, Potenza falls, then Taranto, then Salerno, then the rest of the kingdom. The Savoyard-backed National Guard is wrecked and by the 1870s the Kingdom of Two Sicilies is an internationally recognized country once again. Allies include Austria, Russia and the Papacy (the succesful independence struggle likely butterflies away the Roman annexation 1870), while Spain is considered a good friend. Odds are Carmine Crocco is going to be either a general in the new kingdom, or a provincial governor of Basilicata.
2) Kaiserreich scenario. We all know this.
3) The Soviets and Tito's partisans join with the Italian partisans in northern Italy and, eventually, a socialist republic is declared in Tuscany and the rest of the north in the post-war period. Now here's where it gets really really iffy. The Allies decide to divide their occupation zone in two, splitting the central parts of the country from the south. The territory of the former Papal State is assigned to the Pope, where he remains on paper a figurehead, while in the south the people are given the choice on whether to be a republic or remain a monarchy under the Savoyards. The latter case is more likely but I don't exclude the former either. If the monarchy is chosen, there are some chances that it's going to end up like Cuba under Fulgencio Batista, and there are some chances that the monarchy and every political force that back it (Church, landlords, Mafia) will leave in a rather bloody manner. The country may or may not return united if and when the Cold War ends.
4) This one is also a bit iffy, it requires a change of mentality among southerners. At this moment autonomism or even outright independence are associated with the anti-Southern sentiments of the early Northern League. Well, for whatever reason, the idea that Rome and the north are the real burden on the south's development becomes widespread (a reversal of the northern separatist idea that the Rome and the south are a burden on the north) in the near future and parties advocating some form of autonomy begin to take root. As the European Union begins to fracture and countries like Spain and Belgium see a resurgence of their regionalisms, Venetian and Sardinian independentists also gain traction in Italy. The domino effect eventually touches Southern Italy and, as ethnic tensions between Italians and immigrants reach their peak, the Euro disappears leaving behind several extremely weak currencies (some time will be needed for them to stabilize, and Italy doesn't have this time), unemployment skyrockets and buying power sinks, the first armed revolts take place. Meanwhile in the previous years the government also had the good idea of re-instituting military service (not unlikely, there's been a discussion about it for years and our ministry of domestic affairs is in favour), which means that these regionalistic ideologies eventually make their way into the army. The situation reaches the boiling point, the army fractures along regional lines and the government becomes next to powerless. Eventually Italy ends and the south reacquires its independence, most likely as a Mafia-backed rogue state for the first decades.