The Sassanid-instigated Siege Of Constantinople pitted the Eastern Roman Empire against a coalition between the Sassanid Persian Empire in the eastern side of the Bosphorus and the Avar Khaganate in the western side. In the end, the byzantines managed to hold out.
But what if the perso-avar alliance had won and managed to destroy the Byzantine Empire? Say, by having the persians acquire a naval fleet before attacking or something like that? What would the effects be?
Would the sassanids restore their empire to achaemenid borders? If so, then for how long? Would the Avar Khaganate survive?
What about the remaining territories under the byzantines? Would an eastern-roman-empire-in-exile be created somewhere around Tunisia or Italy?
What are the effects over islam, here still under Muhammad?
As things stood, the Persians already were on the rough edge of their logistics and could not press onwards. Assuming you get a lucky upset victory (not easy as they didn't have the siege capability or coordination to challenge the heart of Byzantine power in a desperately motivated last stand struggle), you'd have a successor state spring somewhere else from the remaining territories (Africa, Greece, what little of Italy and Dalmatia was still Byzantine) and probably hang on and recover some territory when the alliance of convenience fails and try to retake Constantinople from any Persian satrap installed there.
Well, maybe the persians could keep the city and Thrace with a competent enough Shahanshah. Khosrau II was very competent from what i hear, maybe he could estabilish a stable sucession without the massive defeats he suffered at the hands of the byzantines.
If anything, this would overstretch Persia even more than OTL, and when Islam comes, the Sassanids will fall even more swiftly, except this time, Constantinople will become a Muslim city hundreds of years early.