OTL, Dante's influence in bringing up the issues of a common Italian literary language and the influence of Florentine writers like Dante, but also Petrarca and Boccaccio resulted in the slow but sure acceptance of the Tuscan dialect of Italian as the standard form of the Italian language. However, during the Quattrocento and the Trecento, various other Italian dialects had reached the literary form, and indeed there are clear indications of the existence of a sort of Veneto-Lombard koine.
Your challenge would be then to have this form prevail, totally or just in the north at least, over the Tuscan prestige dialect-to-be, and what this dialect might look like closer to the present.
My own personal idea is that if the Visconti conquests of most of modern northern Italy had not been reverted, and the Po valleys had been largely unified under a single dynasty, that this would have been easier, provided that the Visconti had decided to favour Lombard (in the mediaeval sense of the word) authors, resulting in the development of parallel prestige dialects in central and northern Italy.
Your challenge would be then to have this form prevail, totally or just in the north at least, over the Tuscan prestige dialect-to-be, and what this dialect might look like closer to the present.
My own personal idea is that if the Visconti conquests of most of modern northern Italy had not been reverted, and the Po valleys had been largely unified under a single dynasty, that this would have been easier, provided that the Visconti had decided to favour Lombard (in the mediaeval sense of the word) authors, resulting in the development of parallel prestige dialects in central and northern Italy.