So lets assume that the 1848 revolution was a success, Hungary is independent, but lost Croatia and aformentioned territories. Croatia is not important in a sense if the hungarian leaders are willing to loose something its that. However Burgenland and other nortwestern terrotiories will create revanschism in Hungary towards Austria. So most likely Hungary will ally itself with any potential enemy of the Habsburg state to regain Burgenland and the rest. So this makes an alliance between Hungary and France/Piedmont, or Prussia later very likely - I assume that he will regain those terriotires in one of these coming conflicts.
OTOH home affairs of Hungary would start off as very different than OTL. Kossuth was on the opinion that a consensus should be reached with the national minorities. As we know from OTL the wast majority of the hungarian politicans dont agree with this - however Kossuth has tremendous prestige and is immensly popular by the people so it would be very hard to go against him. I also think that this Hungary would be much more democratic - based on Kossuth's views I see a much wider franchise than OTL from the get go. But most likely still a constitutional monarchy.
As for Serbia and Romania. Hungary has a headstart and not only in independence. There is a long standing hungarian elit with lots of money and political experience. Hungary was a more or less autonom state under Austria and has inherited much more efficient practices from Austria than Romania and Serbia from the Ottomans. Its economy is already more developed - though dont having a port will be a problem. OTOH it can develop its own industry without having to compete with the already established and much more developed Austrian and Bohemian lands - one of Kossuths most famous sayings was: "Without industry the nation is a half armed giant" - and he did make some small steps in this direction OTL. Militarily Hungary has easily defendable borders - especially to Romania. Later it can always ally with Bulgaria. Unlike Austria-Hungary Hungary is not a Great Power so can dabble on the Balkans without necessating Russian intervention.
Though Hungary will be at the end of the century be a puppet of the German Great Power. Hungarian leadership end the elit, including Kossuth was very Russophobe - and seeing planslavism growing influence in Saint Petersburg with a good reason. However their only hope against Russia is to align themselfs towards Germany - even if Russia too is allied to it.