That's easy, actually:
Sure, it's a Kingdom of Upper Italy instead of a proto-Padania but, close enough.
An alternate version of the treaty that kept Tuscany independent and let Piedmont-Sardinia absorb Modena and Parma in exchange for the restoration of an independent Venetian state would've created borders closely following language families: a Gallo-Italic (+ Sardinian) Kingdom of Upper Italy in the Northwest, a Venetian state in the Northeast including Rhaeto-Romance but culturally quite similar Friuli, a Tuscan state, a Central Italian state, and a Southern (Neapolitan/Sicilian) state.
The establishment of some sort of confederation could be enough to keep this version of Italy from unifying and, with time, the various kingdoms would've identified with a different historical substratum, while keeping a common Italic/Roman identity on top.