With any POD between 500 BCE and 500 CE, have the Balkans be almost entirely Greek-speaking today. This should include everything south of the Sava and Danube Rivers; bonus points if Pannonia and Transylvania are also Greek-speaking. Double bonus points if there is little to no Slavic or Romance presence in the region, instead having Paleo-Balkan languages such as Dacian, Thracian, and *Albanian survive as minority languages in remote areas. But Greek must be the language of the majority.