More Successful Sassanid Persia? Rome Restored From Gaul? Check out "The Darling Of The World"
One of the causes for the decline and fall of the (Western) Roman Empire is the revival of the old enemy Persia under the vigorous Sassanid dynasty. This gave Rome a major military threat to the east at the same time the Germanic tribes were growing larger and more organized and the weaknesses of the Roman imperial system (namely how the armies could make emperors in the provinces) were becoming apparent. However, although the Sassanids could hurt the Romans, their richest province Mesopotamia and their capital Ctesiphon were close to Roman Palestine and Anatolia and so the Sassanids got the worst of it more often than not. And it was the Romans that ultimately survived in Constantinople while the Arabs overran Persia.
But here's an alternative timeline "The Darling of the World: A Persian TL." This time around, the Sassanids do even better, something that combined with the Crisis of the Third Century has the following effects...
*The combination of the betrayal of the Romans by the precursor to the Palmyrene Empire after a series of Sassanid successes deprives Rome of the riches of the East, especially Egypt's grain supply. This in turn leads to a series of usurpations at the same time as the Plague of Cyprian. Taking advantage of the weakening of Rome, the Goths push through the borders earlier than in real history and establish a new kingdom in the Balkans, putting the boot in on the Romans when someone manages to raise an army to contest them. And then the Alemanni invade Italy itself...
*China is reunited after the fall of the Han by the Wu Dynasty rather than the later Sui. The Wu implement the imperial examination system much earlier than in real history and expand into Central Asia much like the later Tang.
*Although the timeline is meant to showcase a more successful Sassanid dynasty, the Persians' various political problems--too many squabbling princes, a powerful nobility, etc. The Persian emperors who crippled their Roman rivals don't have long to enjoy their victory before they start fighting among themselves over the spoils. And the Palmyrenes who helped the Sassanids bring the Romans low start thinking they deserve more.
*Even though Rome itself falls much earlier than in real history, one of real history's breakaway states--the Gallic Empire--manages to hold the Rhine and rides out the storm. Not only that, but they have the loyalties of Britain and Spain. With the armies and resources of these realms, they're able to (eventually) launch a reconquest of Italy, like a western Justinian centuries earlier.
*Ethiopia becomes the world's first Christian state.
Although the author hasn't updated it since late July (a little over two weeks at the time of this writing), there's still plenty of stuff to chew on in the meantime.