With any PoD after the formation of the HRE (so a nice thousand years or so), have an italian state similar to OTL prussia in its militarism and stately procedure. Bonus points if you can get it to form somewhere not in the peninsula itself but in some remote corner of the italian world (like OTL prussia was formed at the northeastern fringes of germany).
Where would this state be? how will it interact with the maritime republics, the renaissance and the constant meddling of the french in italian affairs?