During the reign of Justinian I, the Eastern Roman Empire reconquered Italy, Sicily, Dalmatia, Sardinia, Corsica, the Balearic Islands and North Africa. Eventually these would be reorganised into the Exarchates of Ravenna and Africa.
Now, I'm not exactly sure when the Exarchates were established, but what if the ERE instead decided to reestablish the Western Roman Empire with its capital at Ravenna (or, being reduced to Italy, Illyricum and Africa, somewhere more central, like Neapolis/Naples or even Sicily.)?
I know the Lombards were a major problem, much like the Slavic peoples were in the Balkans around the time of the last Roman-Persian War. Perhaps if the WRE was reestablished, an emperor similar to Basil II would be able to regain control of Lombard-controlled areas, again, like Basil did with the Bulgarians and slavic-controlled Greece.
But perhaps I'm just rambling here. If such a possibility was to occur, it would have to be after Justinian, since I'm sure he didn't want any rivals.
If anyone has any thoughts or would like to discuss this possiblity I'd be more than happy to listen.
EDIT:
Possible POD: Justinian I's successor Justin II had a brother, Marcellus, who had served in the Lazic War against the Sassanids and was appointed by his father to lead an army against the Bulgars in 562. He is last mentioned in history being involved in the accession of his brother. Perhaps this ATL could come about by Marcellus who, jealous of being overshadowed by his brother, flees to the west and proclaims himself Emperor of the Western Roman Empire. Justin tries to use the Lombards against his brother but is distracted by the Avars and Persians. In the end, Italy is still overrun by the Lombards but Marcellus retains his title and is recognised by Justin as Western Roman Emperor in exchange for promising not to march on Constantinople. This area of history is not my strong point, so I'm not sure how ASB this situation could be.