What makes this worse is the fact that the Holy Roman Emperors were already nominally considered Kings of Italy at the time, having the Visconti take over Italy is surely going to lead to a war of succession. Think an earlier Italian Wars.
While it would be a good way to prevent France from going to deep into Italy, and would counter Aragonese Naples, I don't think Italy would handle having to deal those two plus the HRE...
True, but at the time France was about to enter the final stages of the Hundred Years' War, Aragon had not yet gained Naples and the HRE was preoccupied with a variety of other issues. They might have to deal with them later, but not all of them at once or straight away.
This Italy might also be able to gain French, Papal and Neapolitan friendship (or at least neutrality) by supporting the Angevins in Naples later on, so everyone teaming up on them is not really a given.
Helping Sigismund out against the Ottoman Empire and the Bohemian Hussites might also make the Emperor somewhat less inclined to hostile activities against them.
Doesn't it all depend on much strength the emperor has, and how many of the lords really care about Italy to help the Emperor take it back?
Everyone but the Archbishop of Cologne (he was Arch-chancellor of Italy) would probably feel reluctant to help the Emperor out with what they'd regard as a quest to strengthen the position of the Emperor.
It's not without precedent for someone to be elevated; the kingdom of Bohemia was one example. Charles the Bold was also angling to get a crown; he didn't succeed, of course, but it was something that many people were considering.
Both valid examples, but the situation is somewhat different here seeing as it's not as much giving a duke a royal title as it is giving away one of your own crowns to another ruler. The new king would still be an Imperial prince of course, and nothing would change in terms of how much authority the Emperor would have in Italy, but it would probably still be a scary prospect to most Emperors.
That's the thing though, the Minor Powers preferred a Distant "Emperor" to a local King. When the Emperor Comes Knocking, and he will, the Various Small Lords of Italy are going to join up with him, because having Italy 'Ruled' by an Emperor who is in reality stuck on the other side of the Mountains means that they are the actual powers.
Any Kingdom of Italy in such a situation is going to have to fend off threats from every direction just to survive, and even one significant loss is a permanent defeat. Even if the Emperor can't establish permanent control of Italy, he doesn't need to in order to kill this upstart Kingdom.
Sad, but true. The best bet would be to win the friendship of the Emperor somehow, which is far from impossible if they can maintain their position long enough. Luckily there were plenty of opportunities for both at the time, so the situation isn't entirely hopeless.
I think a surviving Gian Visconti would necessarily include Florence and Bologna, but definitely wouldn't include Savoy. They don't want to mess with the French.
They might attempt to expand into Piedmont eventually, but I agree.
Expanding Visconti territory over the other city states in Lombardy and Tuscany is plausible, but obtaining a crown is much tougher. And no matter how good their relations with the papacy might have been in OTL, the incumbent pope would recognize the obvious threat of a unified Lombardy with its own king.
Consequently I have my doubts that any Visconti wanted such a poisoned crown at all, but if he did, I can see only one way for it to happen. The incumbent King of the Romans leads an army against Milan on the way to his coronation by the pope (bringing the Iron Crown of Lombardy along with him, as well as the imperial crown itself) and is defeated and captured. The price of his release is recognition of the Visconti ruler as king of Italy and handing over the Iron Crown. He's welcome to continue to Rome (with a Visconti escort) for the imperial title.
Wikipedia claims that "Gian Galeazzo had dreams of uniting all of northern Italy into one kingdom, a revived Lombard empire" but I don't know how literally the kingdom part was meant.
I like the idea of forcing the crown from the Emperor, but the long-term consequences would be potentially nasty.
I wonder if stability in Italy would result in the tradition of being crowned Emperor in Rome lasting longer. Italy and the Emperor would of course have to be on friendly terms for that to happen, but it's an interesting prospect nonetheless.
This IS the same Sigismund of the Nicopolis crusade, yes (a failure, but it was hardly an empty threat or angry letter)?
And the Habsburgs would certainly respond to someone usurping one of the imperial titles.
The emperors are not just going to ignore someone claiming to be king of Italy.
They might succeed. They might fail. But they are going to respond with more than angry letters and empty threats.
I was referring to how he treated the situation in the Low Countries. He was extremely hostile to Burgundian expansion there, and wrote a number of very angry letters, threatening to ally with this or that power if Burgundy didn't hand over all their Imperial territories to either him or this or that Imperial prince. Suffice to say, he was not really taken seriously.
Italy is closer to his powerbase though, so the situation isn't exactly the same, I suppose.
Good point though. Imperial friendship would be a necessity for such a state to survive. Forgoing the crown and just remaining the duke of Milan etc. might be a sacrifice worth making to keep Imperial hostility off their backs. (The Emperor still probably wouldn't be too comfortable about a united Italy, but at least he wouldn't be actively hostile or anything. Probably.)