That's a tough subject, there really wasn't an italian nationalist idea for quite some time, even when the Risorgimento came, you could argue it wasn't as popular as you would expect. When you talk about union at that time(1400-1800), you can talk about the re-establishment of Rome, which is a-historical, but to all citizens of the Italian peninsula it made some sense, since people kept referring to it. Specially after Byzantium fell. (It would be really odd though)
Its better to just conquer everything and have all the cultures accepted, its more real and true. The kingdom of Italy, is a NORTHERN European creation, heritage of Charlemagne's time, it has nothing culturally 'Italian' to it.
The fact that the modern country is called Italy is just a bad joke history played on us all, due to it being formed by a half-northern monarchy. But the locals never before such times called their land, as one, "Italy".
Maybe there should be a initial decision, for non-monarchies to claim the Iron Throne of Lombardia, and only then the whole Italy.
Like in my mod-mod where you ask the HREmperor for the Iron Crown which gives you a modifier and all the things that used to go to the Italian Nation decisions apart from changing to Italy. With a decision for a country that has the Iron Crown modifier to later tag change into Italy. This could be modified perhaps so that the initial decision to get the Iron Crown does not change your government type, but the Italian decision would them change to a monarchy. Or have the option to become ITA only for monarchies who have the Iron Crown modifier.
It seems almost like having the nation "Italy" being able to be formed before some point in the eighteenth century, it doesn't make much sense given how the cultural and views of the area were at the time. What did they call the territory that is modern day Northern Italy during the early fifteenth century?
Should we have two kingdoms of Italy then ? The current one is indeed the medieval Regnum Italiae. By definition, it's a kingdom, and its area is northern Italy.
Given that Lombardy refers too much to the region around Milan... so much, that the Austrian possessions after the Congress of Vienna was called "Kingdom of Lombardy-Venetia", that i'm not keen on calling that the unification of the northern half of the peninsula "Lombardy".
Possibly i could need a more proper flag than "il Tricolore"... but i currently have no source.
Now, if it's so important, we could have a second Italian tag. It would require the dismantlement of the HRE or that the Emperor relinquishes the Iron Crown.
What about how I suggested and have the request not actually change the tag over. So it's like the country is asking the Emperor to give back the cultural heritage that is the Iron Crown. This doesn't mean they are claiming the crown as a monarchy however. Just that the HREmperor is recognizing their claim to the region. Maybe the country is just putting the crown in a museum . Then if the country wishes later they can change to a monarchy led Italy. Taking the crown out of the museum and bestowing it on the monarch!