I think Hungary's main issue is that anyone who could potentially benefit from a larger, friendly Hungary has sentimental reasons to oppose it; for example, Russia could use a friendly buffer state across the Carpathians, but Hungary's non-Orthodoxy and non-Slavicness offend Russian sentiments. Similarly, a friendly Hungary could massively help Prussia in any German unification, but having overthrown their monarch the Kings of Prussia are doubtful to support them. The only natural ally for Hungary is France, and France isn't in much of a position to help Hungary against the Serbs or Romanians. IMO, the best scenario for Hungary is to do better at integrating the Slovaks, give the Serbs everything they can afford to, and focus on keeping Transylvania, all of which would become moot with any defeat in war. Even with this, the development of the Yugoslav identity would have to be curtailed in order to give Serbia a bigger foe once the Turks are gone for good, but if Hungary and Serbia stay friendly they could form a Balkan axis of power with Greece, perhaps with a railroad from Budapest to Salonica through Belgrade.Tbf Hungary/Russia/Bulgaria/France/Ottomans Vs Britain/Prussia/Austria/Greece/Netherlands would be fun, with Hungary taking Serbia's place and Russia coming in to help Hungary while Prussia comes in to help Austria. And the war ends with Prussia marrying a female Hapsburg and forming the United Kaiserreich would be fun.
Also Hungary being a regional hegemon makes no sense. Greece-Serbia seem like the ones that would make the most sense.
On other things if things gone differently I could see a united Scandinavia forming unlike otl. Scandinavianism is a thing after all, and I could see them forming into a single nation if they get lucky.