This Italy even after the plague would still be a major power. The amount of damage the Gothic Wars did to Italy left it scarred and a shell of itself. Large cities like Milan were razed to the ground, the aqueducts of cites like Rome were destroyed which saw people fleeing the cities, the entire Roman governmental system that was left intact by the Ostrogoths in otl was in shambles. The actual Roman army was also exhausted and the Eastern Treasury was near bankrupt.julius Nepos is never overthrown by Orestes and has a decent reign where the situation in the west solidifies. There’s no great reconquests besides retaking a few nearby places like the Mediterranean islands and southern Gaul. The next couple of decades are nice and the Nepos dynasty basically does what odaecer and theodoric did when it comes to rebuilding Italy and it’s institutions. Things went south for the WRE (again) when the plague hit Italy hard and left it open to the lombards (but a decade or two later than iotl). The emperors appealed to their eastern allies but they got little by way of help as the eastern empire was stretched thin.
How would the Eastern Empire be stretched thin here? The financial burden from the added war is removed along with the soldiers dead from the fighting. If anything the plague would be a big hit, but the Eastern Roman army would still be intact.
The Lombards only migrated to Italy after the Gepid Kingdom was destroyed which created a power vacuum that saw them move to the devastated Italy in otl. With an intact Italy that rebuilds under Western Roman leadership under the Nepos dynasty, the Lombards wouldn't have had a chance.
This idea seems very ASB. This is something that Justinian would never really do. The major reason why he wanted to invade the West was because there was no more Emperor and he saw it as his duty to restore Roman rule in the West as the next Constantine. He also hated the fact that Italy and the Pope was under the thumb of a heretical Arian King. The Roman Senate had basically been in talks with Justin I in the hopes that he would depose Theoderic. But with a Western Rome in power, Justinian has no cause or reason fro war. If anything he might focus on Persia.In this crazy TL Justinian still takes NA (and gives some money and cheap grain to the west in compensation) but mainly focused on fighting the Persians. He won and cemented an advantageous peace that lasted decades but could not send much aid to his western allies. The western emperors lose about as much territory as the East did to the lombards iotl and barely hang on for the next 150 years.
When the East tried earlier in otl to retake Africa, it was done in understanding that the lands go to the Western half of the Empire. North Africa was part of the dejure lands of the Western half of the Empire.
How does the West fall so easily? The Eastern Empire without the Gothic Wars is very strong compared to otl even with the plague. Without the 20 years long total war fighting between Persia and Rome after Maurice's death, the Eastern Roman Army would not have been bankrupt and sapped of manpower. Plus there's no way that the Pope would really readily accept a Frank becoming Roman Emperor. The native Romans wouldn't accept this. Odoacer was of both Germanic and Roman origin and yet he still didn't dare proclaim himself Emperor. So this alternate Carolus Rex would be seen as a usurper, and would invite war with the East.he western emperors lose about as much territory as the East did to the lombards iotl and barely hang on for the next 150 years. At some point the lombards decided to rid the world of the Western Roman emperors once and for all. They attack and in the opening battle kill the emperor and his heirs. The pope and senate ask for aid but the East is weak (same as in iotl) so they ask the Franks for aid.
Where would the core of this Empire be? Italy here is more intact than in otl. So it would be appealing to setup shop here. Charlemagne was also removed from the fall of the Western Roman Empire by 300 years. The Carolingian Dynasty only rose to prominence because of Charles Martel fighting at the battle of Tours. But without the Gothic Wars, the Eastern Empire is strong enough to resist the Rashidun Caliphate along with the Sassanid Persians since they aren't bled white from civil war. The Franks also divided up lands among sons who fought each other. If Western Rome is still intact, then the endurance of Latin Roman culture is stronger with the actual heart of the Roman world still intact meaning that French might just be a dialect of Latin in upper Gaul/Germany rather than the very divergent language of otl it became. The Franks would also likely be rapidly assimilated into Roman culture because there was likely only a couple hundred thousand of them compared to the many millions of Gallo-Romans, Illyro-Romans, and the Romans of Italy still living within the Empire. So the Franks would be very Romanized kinda like the Visigoth were in otl but to a higher degree. I think the comparison would be like in China with how groups like the Mongols or the Manchurian Qing who invaded and conquered China became Sinicized and assimilated into Chinese culture.In return for becoming the emperor Charlamagne must defeat the lombards. Which he does with great joy as being the emperor is a big deal. Thus the HRE is a continuation of the WRE as Charlemagne was granted the imperial purple by the remains of the western government.