Do you mean for Venice to remain independent alongside a united Italy? If so then yes. There are a myriad of ways this could happen depending on the timing of the POD. For an independent Serene Republic it would have to be before the Napoleonic period (unless you count the Republic of San Marco). Probably just have Napoleon keep the Republic going - with some reforms. With some capable rulers it manages to stay out of Italian unification. It could even keep Dalmatia and Ionian islands. Its a neat and simple solution. I can't see Venice being a major power without a much earlier POD.