I'm rather fascinated with the potential of the Kingdom of Hungary/Budapest had they not been defeated at Mohacs by the Turks. They actually conquered Vienna at one point and basically forced the HRE out of meddling in their affairs. If Matthais Corvinus had lived longer and had an heir that he was able to get ready in time to rule Hungary, they might have remained the superpower of the era for generations more and the Black Army could have been maintained, which would have likely enabled them to hold off the Ottomans the following century. Like most Kingdoms/Empire that fall it was the collapse of centralized power after the death of a strong king (in this case due to lack of a legitimate heir) and the rise of the nobility which led to the dissolution of the standing army (which was the king's main powerbase) and crippling of the kingdom for their own selfish desires for power.
It is mindboggling how stupid and venal the nobility in Hungary was throughout the centuries, all because they didn't want a strong king who would crush dissent among them:
en.wikipedia.org
I wonder if we can draw any modern parallels....
Anyway in terms of a Hungary superpower that remains powerful it would dominate the Balkans, Bohemia, parts of all of Austria, and even parts of Silesia. They also intermarried with the Polish, so assuming that state dissolves roughly per OTL Hungary would get all that. They'd basically be the early Habsburg empire without most of the pesky HRE issues and German nationalism problems and vastly more centralized and modern as a result. Their biggest strength for quite some time was their surprisingly modern bureaucracy and taxation system, as well as professional standing army, which was a novelty in that era.
A Hungary that defeats the Ottomans and turns them into the 'sick man of Europe' a few hundred years earlier would change all of European history, probably for the better if the Habsburgs didn't extend themselves into Hungary (as a result of the battle of Mohacs specifically). It would also ensure the Hungary is set up to be one of the great powers of Europe, maybe on par with France of that era, would shield the HRE from the threats from the Balkans (woe be to France and Poland and probably the HRE peoples themselves given the religious wars of the period), and if it could politically modernize with the times by the 20th century might well be a colonial power in the region like Venice was at one point, conquer Istanbul/Constantinople, and become one of the first oil states thanks to controlling what would be modern Romania, the Vienna basin, and Galicia.
Considering the resources of the region if united in one political entity (and assuming it isn't torn apart by ethnic movements like the Habsburg empire was, which certainly could have been avoided if a centralized Hungarian state was able to push culture and language on the empire the way Paris did on what became France. France being of course a multi-ethnic/linguistic state until the various region had the concept of 'France' and Parisian language imposed on them) it could be a massive industrial power, potentially greater than Imperial Germany if Budapest was the one selling oil to them. It would be rather interesting if Budapest was the senior partner in a relationship with whatever state exists in German Europe.
Budapest would then be an imperial capital on par with if not greater even than OTL Habsburg Vienna, Berlin, Paris, St. Petersburg, or London. It would be interesting to see if they ally with Berlin (assuming that is the German/HRE capital), Paris, or St. Petersburg. Certainly a Budapest-Baghdad railroad would make for an interesting ATL and one that might have even been possible in the 19th century depending on political/military developments.