This one's for Faeelin. Some ideas stolen from some older TLs around here.
In this world the Vandal invasion of North Africa failed to come off, [1] and a western Empire keeping hold of its resources and navy was in better condition to hold onto at least the lands around the Western med. Part of Spain was held against the Suevi and the Visigoths (the Suevi would eventually ally with the Romans against the stronger Visigoths) and without Odoacer's takeover of Italy the Eastern Romans didn't push the Ostrogoths to invade Italy (they would remain in the Balkans, ruling over Dacia and Illyricum south of Noricum until the era of the Avars.
The eastern Empire has had troubles of its own: the whole Justinian mess was avoided, but that just gave it an opportunity to get in another round of exhausting wars with Persia, there were still plagues, and religious disputes between Constantinople and Egypt & Syria still festered, so when a combination of overpopulation, opportunistic state building, and instability kicked off by Ethiopian expansion brought about a non-Muslim but still energetic Arab "surge", both the East Romans and the Persians suffered serious losses. Fortunately, it looks like the Arab expansion may have been a flash in the pan, and the Emperor in Constantinople is pondering whether to try to reconquer Syria or maybe do something about those pesky Goths.
There have been efforts to dynastically reunite east and west, but they've fallen through: with the western and eastern Emperors effectively the heads of their churches (the Bishop of Rome is still only first among equals in the western church) and their clergy holding some divergent ideas on the nature of the Divine, a religious element has entered into the already existent Latin-Greek split, although things are still fairly cordial at this point.
With the Mediterranean still a unified economic area, the economies of places like southern Italy and Mediterranean *Gaul are doing better, and there is a denser network of Maratime trade. Of course, it's a bit unclear as to whether southern Spain and North Africa are really economically doing better under Romans than they would be OTL under the Muslims, but at least things are politically a bit more stable. Even in the North, the Dark Ages are a little less dark.
Italy was still a target of Germanic invaders, with the Alemanni, which OTL just got folded into the Frankish Empire, establishing themselves in the north: the western Emperors moved to the safer location of Carthage, behind the screen of their navy, although they never entirely lost southern Italy. In Spain, some three hundred years of off and on conflicts finally ended in victory over the Ostrogoths, which proved in some ways a bit of a wash, since by the 770s it looked like this would only lead to the Franks replacing the Visigoths as the Empire's principal bete noire...