So, assuming (most of) Justinian's Empire manages to get consolidated back into Rome, what are some of the possible effects of this?
The Empire would include most of Italy (leaving a small quasi-vassal Lombardy in the Northern half), North Africa up to Tunisia, almost all of the Balkans, and of course Anatolia, the Levant and Egypt. It would be THE Mediterranean superpower, even more so than OTL.
Eventually, the Empire in Italy would recover and stabilize under Roman rule, and pay more in taxes and manpower than they cost to maintain.
Some possible effects on population:
-I'd imagine most of Southern Italy would be repopulated by Greeks. It may even be possible for a Roman Emperor to "restore" the city of Rome itself with peoples from Anatolia.
-Greek would still definitely remain the primary culture of this nation, just like OTL. EDIT: By culture, I mean language. Culturally, Rhomanian Greek is Roman by this point.
-With a focus on the west, is it possible that Armenia might be lost to the Persians? Or, perhaps more likely, it would end up as a buffer state between Rome and the Sassanids
On religion:
-Could Chalcedonian remain the dominant form of Christianity in the west? There is no Pope (at least not like the OTL Popes) of Rome, so all five patriarchs would be under Roman rule and might be seen as simply too Roman. In this case, could heresies like Arianism take on a dominant form in Northern and Western Europe?
-So let's assume some other religion rises out of Arabia, either an ATL Islam or an eastern heresy such as Nestorianism. With Italy providing for a more stable Empire and assuming this religion did not rise right after the Empire was exhausted, they should have very little trouble putting it down, correct?
-Persia, is likely to be whatever religion Rome is not. What is more likely to happen, for Zoroastrianism to stay in power,for them to be grabbed ahold of by Nestorianism or some other persecuted Roman heresy, or the ATL Islam?
On Culture:
-Obviously, the Empire is not going to be moving its capital to Rome anytime soon, but I could see Constantinople not being
quite so important to Romans as it was OTL. Especially if cities like Alexandria, Antioch and eventually Rome could rival it. Perhaps it may be viewed as an important capitol, but not quite as a religious and Roman symbol? (i.e how most others view their capital, instead of like New Jerusalem)
-Would Latin remain so important to both the Romans and the Western Europeans? With Rome so strong, it's doubtful anyone will try to claim to be Western Rome (HRE), and the desire to imitate Rome might be lessened with an existential Roman threat on their borders all the time. Could there be no lingua franca of Europe, or could it even be a completely different one like Greek of Frankish

?
And that's just some of the things I am thinking of. Obviously, such a PoD would have MASSIVE butterflies, but we are just looking at Europe and the Near East during the Early-High Middle Ages.