Milan actually managed to conquer most of Northern Italy, except for Venice, under the rule of Gian Galeazzo Visconti. The incompetence of his successors led to the weakening of the Duchy and, indirectly, to the French overtaking of the peninsula. Maybe, if Giovanni Maria and Filippo Maria are not so mediocre, Milan could stay independent from foreign domination, becoming the Venice of this timeline. Maybe, if the loose alliance between the peninsular states manages to survive Lorenzo de' Medici (whose family will eventually have dynastical links with the Sforza), Italy as a whole would be free from foreign rule and later become a sort of confederation. The italian states were by far the most culturally and economically advanced nations in Europe, but they fell because of military reasons: the fragmentation of the peninsula in various, often warring states did not allow the creation of great armies, and when they were present, they were often mercenary armies.