Here's an interesting thought. On August 28th, 475, Julius Nepos, Western Roman Emperor, flees Orestes' advancing army and hides out in Dalmatia. In Italy, Orestes puts his son, Romulus Augustulus, on the throne. In 476, Odoacer pushes out Romulus Augustulus (exiles him to Campania) and assumes the dignity of 'rex' in Italy. The Western Empire is dead, right?
Well, not quite. Nepos is still in Dalmatia, still claiming the Western throne. Historically, he dies in 480, while trying to organize an attempt to reclaim his throne. He was killed by some of his own supporters.
Let's say he doesn't. He returns to Italy and pushes out Odoacer. Do we have a glorious return of Rome? Hardly. At best, he owns a plot of land around Ravenna. Italy itself has practically gone independent and the Popes have a mind of their own. Zeno in Constantinople had decided on Odoacer and also has the regalia of the Western Emperor in his possession. It would take a lot of maneuvering, luck, and time for Nepos and any successors to carve out anything resembling a Roman empire.
For the sake of this challenge, I'll say Nepos flatters Zeno to no end, deferring to the seniority of the Eastern throne, and becomes dependent on the East for his very existence. Not impossible, as it was the Emperors Leo and Zeno who originally nominated Nepos in the first place. As a puppet, with very little to boast of except a hard-to-besiege capital, Nepos might acquire a loose, titular hold on parts of Italy. But he is no true Emperor.
As for the regalia? Well, if Nepos does enough flattering, he might get them back. I could see some interesting political fights between the "Emperor" in Ravenna and the Pope in Rome as both compete for power in Italy. The barbarian states of the Visigoths, Suebi, Franks, Burgundians, etc. are long gone and aren't coming back.
So now moving forward in time.
1) Justinian and Belisarius. You have a titular Western Emperor. Do you use him for your own ends for the reconquest, or are the African, Spanish, and Italian gains strictly for Byzantium? Another note is, Belisarius's battles in Italy don't wreck the city of Rome and cut the last aqueduct. The city isn't dead yet.
2) The Lombards are coming in the 560s. What would Ravenna do? Beg the East for help most likely. A Roman presence at Ravenna would likely check any serious Lombard progress, especially with Byzantine help.
3) 7th century - the Muslims are sweeping across North Africa. A tiny Roman state in Ravenna probably won't make a military difference. They sweep through Spain (Al-Andalus) and into Gaul. Hi, Charles Martel, smacked at Poitiers, and that's that.
4) Franks - Without a Lombard threat, would there be a Donation of Pepin? Doubtful. What would you need the Franks for? Charlemagne, now this is interesting. There's no crown for him to have, Ravenna's got it. The Pope might use him against Ravenna, however. I think Charlemagne would still rule a great state in Europe, just excluding the Italian portion. I could still see him moving against the Saxons, Aquitaine, and even the Spanish March. But without the crown, the nature of the Holy Roman Empire will be different.
5) And that brings us to 1066. There is a Rome, but it is tiny, vulnerable, and clinging to an ancient dignity that is still there, but barely. It's scared of the Pope, scared of Constantinople, and scared of anything north of the Alps. Yet if you have a Crusader Kings player who wants a glorious challenge, then Ravenna would be it.
