The first step to getting Corsica for Italy is to keep it from France, so I’ll address that.
The most obvious way to prevent a French cession is to either avert or resolve the Corsican Revolution. Genoa only sold Corsica to the French after nearly four decades of war and unrest. The rebellion was tremendously costly to Genoa, and although the Genoese managed to suppress the rebels with foreign help on several occasions (the Imperial intervention 1731-1733, the first French intervention 1738-1741), the rebellion simply flared back up as soon as the foreign troops had left. But the Revolution wasn’t inevitable, and if Genoa had treated the island less harshly or committed seriously to efforts towards reconciliation it could have conceivably returned to Genoese control.
The second option would be to have the War of the Austrian Succession turn out differently. The later years of that war saw a British-Sardinian backed expatriate invasion of Corsica (1745) which failed, and a Sardinian landing on Corsica (1748) which came to nothing because the peace was signed shortly thereafter. Sardinia acquiring the island in this war is, IMO, a bit of a longshot, but it was certainly attempted and success wasn’t impossible. That would put Sardinia and Corsica under Savoy-Piedmont’s control, which isn’t yet Italy but goes a long way towards accomplishing your goal.
An independent Corsica could end up as part of an Italian state, but even though I’m presently writing an independent Corsica TL I’ll be the first to admit that the likelihood of success here is not high. Paoli’s republic was the most successful attempt but it could have only succeeded and avoided the French cession with strong British support, which was not forthcoming. Theodore von Neuhoff had the benefit of coming at a more opportune time, but he would have needed some considerable luck to prevail.
Then there’s the box of historical oddities, plans that were either suggested or actually attempted during the Corsican Revolution but went nowhere:
- Spanish Corsica (supported by the early rebels, briefly proposed by Theodore)
- Maltese Corsica (favored by some rebels, supported by the Grandmaster of the Order, but quashed by Paoli)
- Neapolitan Corsica (see Spain)
- Tuscan/Habsburg Corsica (pursued somewhat halfheartedly by Emperor Francis I, possible with a territory swap)
- Stuart Corsica (briefly proposed by Theodore, extremely unlikely)
None of these are likely to occur but several of them have at least a bare shred of plausibility given the right circumstances and personalities. Some are likely to end up as Italian (especially a Tuscan cession), some less so.
The only real opportunity for Corsica to escape France after the conquest in 1769 would be during the French Revolutionary/Napoleonic Wars, either as a consequence of a more successful Anglo-Corsican Kingdom (1794-1796) or a different outcome of the Napoleonic Wars such that France is forced to cede Corsica in a peace treaty. The likely outcome of that is a British Corsica, not an “Italian” one (unless the cession is to Sardinia, I suppose), but a cession of British Corsica to a newly-formed Italian state may be possible in the same manner as the United States of the Ionian Islands was eventually ceded to Greece.
