f I have to try to predict anything I'd bet on Tuscan Corsica (An international compromise solution.), stronger Tuscany hit and sacked by Napoleon enough to have pre-Unification balance of power not changing, therefore Savoy still unifies Italy.
Now:
- Would they keep New Tuscany?
- Would New Tuscan wealth mean less violent taxation (Therefore less troubles in the South.) for Italy?
1) If the Medici still go extinct as OTL (not necessarily likely. Ferdinando I and Cosimo II both had several sons), the colony could end up in the hands of whomever gets Tuscany. If it's a Spanish infante, I could half see Spain simply annexing "New Tuscany", same goes for an Austrian archduke or French prince inheriting. Even if Tuscany were to regain it's independence (à la Leopold II), it doesn't mean that it's not going to lose it's colonies (since I'm guessing that a New Tuscan success will prompt other (although not necessarily successful) colonial ventures elsewhere).
2) Well, if Tuscany is wealthier
because of this, I could see the balance of power in the peninsula shifting. I'm not sure what Savoy's GDP was, but if the Medici manage their colony with a degree of financial acumen (not sure of this one, Ferdinando I was IIRC, the last Medici duke to have anything to do with the family bank), it's a good chance Florence will be richer than Turin. As a result of being wealthier, Tuscany can afford a stronger army and a stronger navy (not saying they will, just saying they can). Hell, maybe they would even do a Swiss/Hessian thing and rent their troops out to foreign powers. Which means that the balance of power in the peninsula
will probably change.
Now, whether Guiana/New Tuscnay is still generating wealth by the time of Italian Unification depends both on when Italy is united - if Isabel Farnese marries the duke of Modena (as was planned) and only has a daughter, and said girl marries the grand prince of Tuscany, that's already a massive block in north-central Italy under Tuscan rule - as well as the management of the colony. Do the administrators adapt to changing markets? For instance, once it becomes cheaper to get cotton from London (in the 18th century), do the Tuscans knuckle down and look to industrializing - as François Étienne of Lorraine started them doing? - or do the Florentine guilds cling to their privileges and block it. Likewise, do they adapt their crops? Okay, everyone else can get cotton cheaper from London, and pepper from the Dutch, so they stopped buying from us. What are we gonna do about it?. What about coffee as a cash-crop? Or tobacco? Do either grow in Guiana? Or do they just carry on plugging away at the cotton/pepper pairing because that's all they know?