Which made me wonder:
With a POD no earlier than 1st January 1800, what is the most powerful, either mulitarily, financially, politically or culturally, that the Italian peninsula could realistically get?
Italy is kinda out of luck here as all the other major power have been established and consolidated (ie France, Russia, Prusia, Austria, Britain, UK, Ottomans, etc.) Though if you push back the pod to the middle Ages a Renaissance Italy might have easily accomplished this since this was when historically Italy was at its peak potential since its past under Rome. As other European states began to move West for Colonial Expansion and better trade prospects, the wealth and sheer volume of trade from these regions soon surpassed the Mediterranean which had been displaced from its position as the hub of European trade since Antiquity. With all this added wealth and the consolidation of larger states like France, and England, the Italian states were too small to really effectively challenge them. If someone like the Visconti manage to Unite Italy like they almost did in otl, then this would have laid the groundwork for a Renaissance Italian Unification which would have been immensely wealthy and powerful in that era.
Though if you wanna go to the 1800's the best bet is maybe not having Napoleon and the French Revolution as this means that other more liberal and reformer Kings in places like Naples might not be scared of continuing with the Enlightenment and going full reactionary and opposing any reforms.
Though another possibility is that you could have Murat's Kingdom of Naples survive as it almost did happen in otl. In my TL
Imperator Francorum: A Napoleon II Timeline, Murat and Caroline Bonaparte manage to keep the Napoleonic Kingdom of Naples intact after the Fall of the French Empire. Though in my TL there's no Hundred Days as Napoleon I dies in battle as a badass hero of France leading his men at Leipzig.
It almost did in otl, and the only reason why it fell was because Louis XVIII of France forged a letter claiming that Murat had offered his support to Napoleon during the Hundred Days Campaign. Louis XVIII also personally funded an expedition to restore his Bourbon cousins to the Throne. Murat's Kingdom almost made it out of the war and would have been a surviving post-Napoleonic State which would have been positioned to make use of the Italian Revolutionaries. Napoleon's sister Caroline Bonaparte who was married to Murat,was banging Metternich while he was negotiating the terms of the Congress of Vienna and the Concert of Europe. She also betrayed Napoleon in an attempt to stay on Austria's side. Austria also didn't really want Bourbon influence in Italy as the two Houses have been historic rivals which is why they wanted Marie-Louise to get the Duchy of Parma and the Kingdom of Naples to go to Murat. The Austrians were more in favor of Napoleon II keeping France. After all Napoleon II was the Austrian Emperor's grandson whose daughter was the Empress Consort. Thus this would give Austria influence over the regency and given Austria a an ally in the form of a powerful but contained France as a counterweight to Britain and the other Great Powers like Russia which Austria had conflicts with in terms of influence and territory in the Balkans. Austria after all was historic rivals with the Ottomans and had a claim to the Serbian throne and had at one point controlled Belgrade.
If the Napoleonic Kingdom of Naples survive, it would likely continue to reform. It would also likely have a better army and administration since Caroline Bonaparte like her brother was quite a good administrator. Murat was an excellent cavalry commander so he would likely build a strong Neapolitan army based on French military doctrine which at the time was still the premier military power in Europe. She also would like pursue better relations with Britain like in otl where she welcomed in the British, opening up the Neapolitan ports to the British fleet and going as far as to facilitate British tourists and employing British nannies for her young children. Naples would likely adopt a pro-British policy, or at lead ingratiate itself with Britain. This would mean that Naples would receive more direct investment from British bankers and industrialists as the Industrial Revolution kicks off.
Though Sicily would be under the Bourbons and would likely be pissed. Though to be fair, it's also a blessing for them, since they haven't had a King in Palermo since the 1270's during the reign of Manfred von Hohenstaufen. The Sicilian Bourbons would likely be gunning to retake their birthright of Naples as well. They might also be more Pro-French though to gain more military support. I don't see Naples doing much in 1830 as it just barely saved itself after the Napoleonic Wars, and the Reactionary/Conservative Order that was the Congress of Europe quite easily crushed/suppressed the Revolutions of the 1830's. Plus the Pope was also technically the feudal overlord of Naples and the Murats wouldn't want to push against the Pope at this time. Though in 1848 I can see many Italian revolutionaries looking to Naples for guidance rather than Sardinia Piedmont because of Naples being a more liberal power whose rulers were moulded by the French Revolution. This might turn up into a three way struggle between, Naples, Piedmont, and Sicily with the Papal states likely trying to play off all three against each other to survive. Though I would bet on Naples here which would likely be in a much stronger position rather than in otl. It was the largest of the Italian states and was quite wealthy. If the Murats institute French style land reform and conscription they can likley build an effective army to probably take over Italy.
This Italy would likely be a unitary state and likely evolve alongside the Second French Empire as perhaps a Semi-Constitutional Monarchy or even a Constitutionally Absolute Monarchy if the Murats are capable enough to pull that off like Napoleon did. If Napoleon III comes to power like in otl, he might ally with the Murats who were his cousins. This Italy though would have to play carefully as to avoid rocking the boat. If Napoleon III like in otl has his plans to reform the army defeated in the French Parliament, then its likely that Prussia would win, and seeing where the wind is blowing, a Prussian-Neapolitan/Italian alliance would likely see this militarily capable Naples gain more territories. If say Austria collapse like it almost did in 1848 with Hungary splitting off, then it might ally with Hungary against the Austrians to gain control of Venice and likely Istria.
I can see Naples maybe co-opt the Pope into Italy with the Pope being the spiritual head of the Kingdom while the King handles the "mundane temporal affairs" like administration, military affairs, trade, diplomacy, etc. This Italy would be better off than otl because the South's economy doesn't collapse like in otl and the Italian diaspora which saw millions of Italians flee overseas would never happen. Italy would then not have a massive brain drain. Without the mediocre Savoyards, and better governmental system Italy here would be better governed which would do wonders to improve its economy. In terms of colonial expansion, Italy might be able to focus on getting Tunisia and Libya playing up the old Roman conquest of Carthage. Though I can see the Pope perhaps ruling Rome as a compromise with maybe Naples being the Capital of Italy since Naples was the third most poulous city in Western Europe at the time.
Otherwise the best you can do is a POD in the high middle ages, where the Holy Roman Empire both become more centralized, and the Italian half somehow dominates the German half, such as the Emperors make Milan their capitol and keep control over both kingdoms.
For the middle ages, you could have the Hohenstaufens win with Frederick Barbarossa gaining enough troops from Henry the Lion to actually beat the lombard Leage at llegano allowing him to reassert control over Italy. Another POD is that Emperor Heinrich VI lives and thus is able to better integrate Sicily into the Empire after passing the Ebereichsplan making the Empire hereditary while possibly preserving the powers of the nobles which creates a more federal Empire while preventing the nobles from dividing up their lands and further decentralizing the Empire. In ttl, Frederick II would be raised as a traditional Hohenstaufen Emperor, and if he's anything like his otl self, he would likely be the most powerful and learned sovereign in Western Europe since Charlemagne. If Philip of Swabia lives then perhaps the HRE would centralize and Frederick II would rule Sicily as a separate Kingdom with it maybe being integrated with the main line of the Hohenstaufen dynasty later on via an inheritance.
Another possibility is that the House of Canossa continue to survive in Italy keeping the Margraviate of Tuscany intact. This was a huge swath of Northern, Central, and Southern Italy that would likely emerge to dominate the HRE. If they perhaps intermarry with the Salians then we might see a jointly Italo-German HRE with a huge foothold/core in Italy. This HRE would be more Holy, slightly more Roman, and definitely more Imperial.
I think the most significant thing to change Italy's performance is to have Italian unification take place in a way that has buy-in from the Church.
This could likely be in the form of an Italian confederation that gradually centralizes into a united Kingdom of Italy like how the North German Confederation become the German Empire. Or you could maybe have a compromise with the pope where he rule Rome while the Capital would be in another city like Milan (ie the Visconti wins) or even Naples if the unification is led successfully by the Bourbon Kingdom of Two Sicilies, or the Napoleonic Kingdom of Naples led by Murat.
