You need a PoD around the 680s (that is when the Byzantines lost to Asparukh's Bulgar-Slav army in Ongal, starting the formation of a powerful Slavic First Bulgarian Empire) for this to work, and even then you still have the Slavic migrations occuring.
That's a little early. Byzantium still controlled most of the Balkans until the reign of Isaakios II Angelos. After that is becomes difficult, but I'd say still doable until the time of the Ottomans, roughly 1400 sounds about right, though the reign of Andronikos II Palaiologos is also a good time to call the Greek Balkans a dead dream.
By 1500, nothing short of ASB will accomplish that. Slavs in the Balkans outnumber the Greeks by too much, and the chances of Greece conquering the whole area, let alone culturally converting it, are basically nonexistant. Greece controlling modern Greece, Macedonia, and European Turkey, with the possibility of adding Kosovo, Albania, and Montenegro, is about the best you can get. The Danube, let alone anything further north, is a lost cause.