As the tin says, what if Khosrow II accepted the offer Heraclius made in 615 when, per Wikipedia, he “agreed to stand down and was about ready to allow the Byzantine Empire to become a Persian client state, even permitting Khosrow II to choose the emperor.[20] In a letter delivered by his ambassadors, Heraclius acknowledged the Persian empire as superior, described himself as Khosrow II's "obedient son, one who is eager to perform the services of your serenity in all things," and even called Khosrow II the "supreme emperor."”
How long could this situation last? Would Khosrow appoint the person he claimed was Theodosius as emperor? What possible impact would this have on the Sassanids, the Romans and the Arabs?
If we presume this whole history is true (we can't take everything about this war at face value), and assume that with it the Sasaniand get Syria and Egypt (so the dioceses of Orient and Egypt) then the Persians are out for a ride, as it seems they didn't had a lot of administration set in place and in general there was a big questions around.
For religious figures, well Khosrow seemed to be somewhat sympathetic to Miaphysism, particularly the Jacobite school of Edessa (which his favorite wife Shirin seemed to be part of) in contrast to the East Syriac rite of Ctesiphon the Sasanian kings used to support, add the Copts of Egypt in the mix and it is going to be another variable on the table, Khosrow and his court have to deal with several Christian denominations, with the cherry on the cake being that Iran is now a Christian majority country (although Zoroastrianism is held by the majority of the Iranians east of the Zagros), what comes from it it is really uncertain.
For the Arabs, well the Sasanians would be more concerned about the South for a while, as the Arabs have being a problem since Khosrow destroyed the Lakhmids, Rezakhani on his paper on the fall of the Empire makes a case on how the Sasanian overruning the Roman East eliminated the borders of Syria and Iraq, as many Arabs (like the Ghassanids) once bound by old tribal boundaries, began eb and flowing though the Fertile Cresent like there is nothing, if Ctesiphon isn't breaking itself apart as they did 628-632 you could see an unified and effective policy to deal with the southern barbarians .
The south isn't the only problem, you have the north too, the Western Gokturks, arguably the true victors of the Roman-Persian war can attack the Empire on the first sight of weakness, and now it is pretty much the Sasanians' sole job to protect the Caucasus.
