Initially, my question was this: How late can the Roman Empire be reunified before Greek becomes the main language of the Empire? Latin held on until around Justinian's day, and Greek was pretty definitively the dominant tongue by Heraclius's reign.
However, the question isn't quite that simple as we'd have to consider two major variables:
- How much of the west gets reconquered.
- How much of the east is retained.
After all, an Empire that is goes no further than Anatolia in the East but has Italy in the West might very well be fairly Latin in nature after a few generations, even if this is after the Heraclian dynasty, even without other areas like Spain or France.
So, with that in mind, what sorts of scenarios could we envision in which Greek is not overly dominant over Latin (in some form) in a Roman Empire of some form or another?
(don't try to be cute and have another language, like Persian or Arabic be the dominant tongue)