I once toyed with a POD (Venice retaking Cyprus and applying some aspects of Ottoman admin to it's governance as a lab) leading to them maintaining enough of a navy to (with British help, although most historians of the Serrennissima mislike playing that up) keep the French out of the Rialto and Dalmatian islands.Given any POD, have Venice remain a Great Power until the present day. Its form of govt, however, must remain the same as existed at its height.
Sorry if it's been done
Of course, the Aregno gets restored as a House of Commons during this mess whilst the Nobility is seriously shaken up. The longhi/curti conflict is added to by the 'merchants' who still kept a hand in various trades and 'farmers' who aped the aristos on the continent... and the latter were largely ruined (and several were banished as collaborators) in the Napoleonic war.
The council of Vienna grants status quo ante bellum to the place largely due to the refusal of a monarch, but the place becomes increasingly democratic and remains one of the most religiously open lands in Europe (a qadi subject to the Mufti of Candia is a formal office, while five non-catholic families (two Orthodox, two Muslim, one Jewish) have been added to the Golden Book).
Definitely not the Venice of old for all that the Grand Council and Doge continue thier ceremonies, does that count?
HTG