It's difficult, I think, to get the exact circumstances wished for by the OP. That said, I could see something like it happening if the system set up in the late Roman period, the Quaestura Exercitus survives, perhaps in the absence of the rise of the Arabs. Basically, this was rather an odd "province", grouping together the provinces of the Danube frontier with some of the Aegean islands and southern Asia Minor, with the intention of supplying the army on the war-ravaged Danube from provinces sheltered from direct warfare. Keep this system alive, and I'd suggest you begin to provide an incentive for the imperial government to begin to settle Greek speakers from the southern regions of this "province" to the eastern and northern Balkans.
Keeping Greece Sclavenic is probably trickier, given the OP likely requires a surviving and active East Roman/Byzantine state. You can perhaps have larger Sclavenic-speaking pockets survive until considerably later than IOTL, but I think sooner or later its inevitable that the region will be brought under Constantinople's control and resettlement of Greeks from Asia and Sicily will begin: it's also important to note that it's very unlikely that Greek speakers were ever completely extirpated in the first place. I think at minimum, Attica, the Peloponese and the region around Thessalonica will end up Greek speaking.