Many cities in the Balkans could be much more relevant and bigger, perhaps simultaneously with the right circumstances
Belgrade - excellent position, but was a border town for much of history, so it was perpetually destroyed
Smederevo - pretty much same as above
Novo Brdo - Was a gargantuan mining town in the middle ages
Skadar - was a pretty important city and center of trade
Kotor - good, secure location, center of trade, but suffers from lack of available space, if it was bigger it would be very stretched
Trnovo - ancient Bulgarian capital
Drač/Durres - major port city, like all balkan cities declined when the ottomans invaded
Dubrovnik - was a minor trading nation, but could have been more relevant