Firstly I think it'd be easier for your cause if you can keep Spain and France out of Italy early. Once they come in, they don't actually get out, at least until it doesn't matter anymore (see option 3 for this to really apply).
That being said, there are a few scenarios that quickly come to mind with regards to Venice. You can have Venice continue its many successes over Milan during the height of its power under Doge Francesco Foscari, with Visconti living longer and the war continuing in the Republic's favour.
In 1482 you can have Venice's war against Ferrara go better. It was allied with Pope Sixtus IV against Ferrara and her allies of Florence, Naples, and Milan. If the battle of Campomorto is avoided or goes better for the Papal-Veneto alliance, you could be in a position to claim much of the area. Even despite the loss of the battle and the Pope as an ally, Venice still managed to make considerable gains in the war, while upping its prestige.
Or, maybe have Ludovico Sforza of Milan not give France a free entry into Italy, instead requesting the help of Maximillian against Venice. The war can go south for the Empire, as Venice seemed to be quite effective against its forces such as in 1508, and Venice could go on the offensive, taking much of Milan.
A combination of the three actually sounds pretty good IMO. You could possibly have a significantly stronger Venice by the beginning of the 1500s, with a significantly weakened Florence, Milan, and possibly no Ferrara.
Those are just a few. Forgive me, my mind seems to be solely on the war option today...