In this alternate timeline, the Ottomans are far more hostile to Italian traders in the wake of conquering Constantinople. Now being cut off from a large source of their profit, and desperate to make it up, the Doge of Genoa, Paolo Fregoso, accepts Christopher Columbus's plan to attempt finding a route to India. In 1487, he sets off to find the way, arriving to Plana Cays in early 1488(slightly before the southeast passage through the cape of good hope was discovered by Dias) and establishing an outpost. Spain and Portugal are primarily occupied with attempting to reach India through the southeast passage, however, Venice takes interest in the Genoese route, and the two quickly find themselves competing in the new world over trade and resources. Neither is particularly interested in conquest outside of areas that would provide trade benefit to themselves, so aside from much of the coast, the interior of the Americas lies fairly untouched by the Europeans, despite the massive waves of disease and political instability. The natives, for the most part, are uninterested in dislodging the Italian traders, as they provide many luxury manufactured goods and serve as profitable trade partners. One of the most lucrative trade relationships that would develop would be between the Incans and the Venetians/Genoese. The Incans had many valuable trade goods, including gold and rare spices, and also had a large and robust enough economy to reliably do business with the Italians on somewhat equal terms. The Aztecs would almost certainly collapse due to the previously mentioned wave of disease and political instability, and the Italian's vested interest in keeping the extremely valuable gold, cocoa and silver producing regions of Mexico easy to dominate. The already extremely highly developed cities of northern Italy IOTL would swell to be enormous commercial metropolises, and with the merchants of Genoa and Venice having far greater experience than those of Spain, as well as the countries themselves having far better leadership than Spain did in the 1500's, the chance of total economic collapse due to oversupply of gold and silver is much lower. I envision other Italian commercial cities taking part in these activities, particularly Florence, Lucca, and Pisa. Thoughts?
Venetian Incaland
Okay first things first, the Ottomans would be stupid to be as hostile as possible to the Italian City-states considering they still have power. The big question is how do the Italian city-states get ships that are used to navigating the Atlantic in first place, Then how do they keep nearby bases to travel, and what's to stop England, France, Castile/Aragon or Portugal from simply taking them?
Writing style-wise spaces would help. The thing with the Italian City-States is they might not have enough manpower and can be really stretched thin or forced to prioritize Italy if things in Italy goes south like say the Italian Wars. So I cannot see them as a power able exert influence.
You're right about it being improbable, concerning the other European powers, but I mentioned how the city-states only bothered conquering bits of the coastline, so I doubt they'd get very overextended.