I do not believe this topic can begin without discussing what would happen to the cities. They would look, act, and behave, fundamentally differently in a surviving Empire than in the resulting nations that emerged out of it. Since the cities would in time resemble microcosms of the Empire as a whole.
Prior to the War Austria-Hungary had one of the fastest growing economies in Europe. However, they hadn't reached that point on the demographic transition model of a population boom and urbanization. They were close to crossing it, but the dissolution of the State came before it could happen. The nations that came out of the dissolution were ethno-nationalist unitary states and they had no interest in anybody in their cities not using their language. Now Austria and Hungary had their own assimilation policies. In fact Hungary's Magyarization policies were the most successful in their cities. However, the main difference facing the cities this time would be the sheer scale of the immigration. The new urban poor would be flooding the cities from across the empire and I do not think the cities would be able to keep up with it. The assimilation policies would fail. They would be overwhelmed by the volume of people. For example, I don't think it would be unreasonable for a modern day Vienna in TTL to have around or over 10 million people. Are all of those people going to be German? Of course not. Now I do not believe that these cities would be homogeneously mixed. Vienna would have ethnic Slavic, Hungarian, and Romanian quarters (to give a few) and many cities have across the Empire would all be multi-ethnic of various degrees. The key difference is that there is no mono-nationalist government trying to force these people to all be the same and assimilate. To put it in another way, Austria-Hungary had 52 million people in 1914. I could be conservative and say the population of a modern AH would be double that at around 100 million. Yet a majority of those people are going to be living in the cities after the demographic transition is over. And when a majority of people are living in the cities, Austria Hungary may have the same borders but it is a fundamentally a different country.
Organized labor would be fascinating. I think at first labor unions would have a nationalist bent to them and refuse to cooperate with each other because of that. That is self-defeating however, as employers could play nationalities off each other. They may even try to encourage immigration from other parts of the Empire into the city, just so they have a means to do this. Now I am not saying they would intentionally do this (though they might), but if there is a nationality in the empire that is willing to work for cheaper wages in the factories than the ones working for you, why not invite them over? That's capitalism. Perhaps unions would then try to downplay ethnicity and nationality as it isn't helping to organized labor in its goals. Maybe they would take up Austro-Maxism, which was frankly two or three generations ahead of its time to catch on in OTL but could work well here.
https://en.wikipedia.org/wiki/Austromarxism
When people live close together, they are forced to speak to each other. So no matter how cosmopolitan these cities are, they will need a lingua franca. That could be German or Hungarian. I can't say much more than that since I am deliberately ignoring the politics of a surviving Austria-Hungary. As for the various "nations" of the Empire, I doubt those will ever go away, though they will be less all-important once the majority of the population is urbanized. People will still speak their own languages, even across the official borders of the Empire. But by that point the Empire is so big, cosmopolitan, and economically interdependent, I am having a hard time imagining the whole thing failing.
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