Which historical or modern figures in the Balkans do you think truly deserved to lead their country but never got the chance?

It great that you mention those areas because they are in fact the major problems. The solution is not so complicated. The majority of the industrial output wouldn't funnel to one export partner at whatever price they decided it was worth, regardless of the economic viability for the production. Even more so when this partner was the political puppeteer of your state, which meant the low price couldn't be argued so many of those factories were simply working on a loss because they were told to. This industrialization on life support is how most of those great many factories eventually shut down just a few years into the 1990s when the state no longer wanted to bear the cost for running them and got them privatized for cents on the dollar. So, all of the amazing growth and development is an illusion if the achieved industrialization is induced and conditional, which it was. As for the education, I'd say it was decently developed up to secondary education level. Indeed more people got to visit schools, the language was concretely standardized to remove regional variations, and the educators were paid fairly, but this came at a cost by cutting off Western affiliation at the tertiary level. Those renown universities in England, France and Germany that some of the famous Bulgarian statesmen, economists and public figures had attended during 1860-1940 were no longer available. Instead the best you could hope for was going to Moscow and that was only if you had the necessary political connections (i.e. your father was a party member for instance). This (and of course the "removal" of the monarchist elites and highly educated non-communists) resulted in gross incompetence in most economic and societal sectors. I believe many of the effects of Soviet-style socialism (it was not Marxist socialism) would've come through regardless of who was in power - whether Petkov, the tsar, or even a democratic leader. Thing like paved roads, indoor plumbing, electric infrastructure, etc were not invented by the Soviets so that we can be sure in an alternate universe Bulgaria under a different administration wouldn't have them and the rest of the urbanisation and industrialisation