the only thing that comes to mind is a typical Islam-wank
POD 610 A.D. - a storm hits Heraclius' fleet, sinking most of it and killing him.
Phokas continues his incompetent rule and Constantinople falls in 626. The Persians however severely weaken their position with attempted invasions of Greece and Carthage. Maybe add a civil war or two, and when (after the appropriate butterflies create a more aggressive version of Islam) the Arabs come knocking, the entire house falls down. Meanwhile the Avars settle in the Balkans.
By ~700 AD the Arabs have pushed as far west as Morrocco and as far north as the greek coastal areas. The Balkan interior is dominated by Slavs, who eventually convert to Islam.
By 750, instead of attacking Spain, Italy is invaded and more or less conquered all the way to the Alps.
Spain follows cca 820. When the Caliphate breaks up, both Spain and Italy manage to stay united instead of fracturing and, over time, grow predominantly muslim.
Islam meanwhile continues to spread among the Slavic tribes to the north.
With a bit of luck, the Maghyars, if they still arrive in Pannonia, convert to Islam as well.
When the colonial Age comes, have muslim Spain do at least as well as the christian one in OTL and be very thorough with the conversion of native Americans.
If some muslim states get an industrial revolution at around the same time as the europeans, then maybe a great deal more of Africa might turn muslim as well, even more so if they beat Nubia and Ethiopia early in the game.
Alternatively maybe something involving Zoroastrianism becoming popular ?
Getting Graeco-Roman paganism back up to strength with any POD after Julian the Apostate is IMO impossible, and even with everything going right for Julian I'm not sure it has any reasonable chances.
