...aaaanybody?
I dunno? Surviving Western Roman Empire?
Basically, it would be a major ball-ache, even more than the insoluble problems of Karl V. You see, you don't just have to hold it together for one King's reign: you have to keep all these unions together for as long as it takes for the next country to join, and then stop them complaining about the King spending too much time in his
new new patrimony. That's why this is an AHC, I suppose.
I mean, holding
just Iberia together took 40 years to fall apart into complete anarchy (rebellions in Catalonia and Portugal, plots in Andalusia and Italy, etc.) and that may be due to corruption and mismanagement, but a lot of it was down to economic puzzles that wouldn't be solved for centuries - people were only just working out that the amount of gold in a country didn't exactly correlate to its wealth, for instance. So this AHC requires not only a load of lucky marriages and deaths (and the ignorance or antipathy of the leaders of the Great Powers, who definitely didn't want to have their national interests subsumed into a contradictory Imperial whole) but also up to five generations of Great Kings with foresight and industriousness - AND five generations of capable and wise administrators to populate their court and generally make sense of half a dozen different systems of law, customs, taxation, language etc. etc. All this in a system which is predisposed towards the accumulation of wealth and influence in the hands of noble
families, which are by no means the best ways of ensuring that all the major politicians are verging on being autistic savants.
So lets say, for instance, that Juana the Mad marries Henry VIII, and alt-Mary I marries a Valois (never mind the problems this brings to the traditional Aragonese and English anti-French policy) and then that Valois guy inherits the French throne and then the next generation brings Portugal with it and then we finally get Scotland joined up with the next generation, because
for some reason James the Whateverth hasn't been paying attention to what happens when your eldest daughter marries the heir to another country. So this means that Aragon and Castille have been hating each other's guts since 1469, and then they've both resented the English since c.1510, and all of the above have, in turn, detested the French since c.1540, and then Portugal and Scotland have been coming to blows with the whole lot of them, so the whole thing comes to fruition in c.1590, and over the last 120 years, we need everything to have remained completely calm and prosperous, because any crisis will bankrupt this illogical smorgasbord of rivals who don't even share a language
family with one another because of the cost of administration outstripping the income via taxation. And you want another generation to hold this all together, taking us to c.1620. In fact, the shortest time this could come to fruition (+a generation) would be four generations, which would be a century or more.
Essentially, what you're asking for is for the European Union to come several hundred years in advance, and last for at least twice as long as it has done so far, with the added complication of nobody actually wanting it to exist and none of the members having the kind of education system or meritocratic bureaucracy that is required for the EU to be feasible. You need four generations (at least) of Karl Vs, each surrounded by hundreds of clones of Cardinal Richelieu. I'm not saying its ASB, but it would be much easier if the previous 1000 years of history hadn't happened.
