What if Western Rome held on to Italy?

This thread asks the question of what if Western Rome never fell, well, if everything fell except for it's Italian core.

For the purposes of not seeing Rome collapse within a few decades of this scenario going, let's say they lose the expansionalist mindset to focus on their homeland. (Simply because if they kept it a coalition would probably wipe them out, or overextension would kill them.)

How would this Rome play in the great game of chess Medieval Europe was, how would it influence the politics of the continent, and would it have stepped in to delay the decline of Byzantium, would the greek speaking eastern empire be a rival or ally to the latin/italian speaking western portion?

One thing I know for certain is, no HRE, which would have untold effects on Germany and the low countries.

An interesting scenario, especially for an italian as I am.
First, we must decide one thing: if it goes on mixing barbarians with romans, or not. Let's suppose the second possibility is true.
5th Century
First, the Ostrogoths led by Theoderic would not estabilish in Italy but between the 2 empires, in modern day Bosnia and Croatia.
6th Century
Then, Theoderic, who actually served Justinian as a general, would give a good help to defeat the Sassanids, because there isn't any Rome for Justinian to regive to the Romans. Then, the Lombards, migrating to Italy, would be stopped by Rome when entering the Alps, therefore they would mix with the Bavars.
7th Century
With a strong Byzantium, that in this scenario can easily give the "coup de grace" to Persia with Heraclius, we can MAYBE imagine a stop to the Muslim advance, and therefore a still Roman Africa.
8th Century
We can try to see a ever-fighting Byzantium that in this case seeks desperately for allies, and finds them in Rome; while Rome can gain profit from this weakness to invade the Ostrogothic lands and re-romanise them (by doing a genocide and importing Roman settlers), and maybe also to recover Sicily, Sardinia and Corsica from the Vandals. Meanwhile, we cannot have the Papal States because the popes haven't had a key role in keeping barbarians out of Italy, and they remain only religious leaders. Charlemagne, meanwhile, would not have the authority and the power to invade Italy, so he would maybe conquer all of Germany and fight against Slavs in Poland (his advantage in our timeline was to fight against pagans, like Saxons and Al-Andalus).
9th Century
After the split of Charlemagne's empire, Rome would easily take profit by conquering most of France and re-romanising it, while Byzantium can finally defeat the Muslims and conquer Arabia.

Great thread topic, Captain Magma.

Excellent synopsis, Francinze! I find myself with surprisingly little to add. Brilliant break-point keeping the Goths in Yugoslavia (Illyricum) in order to allow Italy to remain in Roman hands.

I really love reading the Alternate Histories here. I never knew there were so many people like me in this regard.

With regard to Medieval political developments in Germany after the close of the Carolingian period, since they are not organized under the HRE, perhaps the larger states swallow up the lesser ones until there are just three or four distinct powers in the region such as what happened in Scandinavia, France, Spain, the Netherlands, Great Britain, the Russias, and even in the Balkans. From this we see the roots of modern Nation-States. Perhaps a single kingdom eventually emerges in Germany during the renaissance just as the monarchies based in Madrid, London, Vienna, and Moscow eventually unified their respective regions into "countries". We know how Prussia finally unified the German states and the after-effects of that state on modern Europe. What if Germany emerged as a nation-state 400 or 500 years earlier?

I can guess that there would not have been a 30 Years War like there was in the HRE during the first half of the 17th Century. But how would the presence of a powerful country, presumably with a typically autocratic monarchy, between Poland and France have affected the course of European history from 1450 to 1850? Would there even have been a Protestant Reformation? In the 1500s, decisions about what religion people would be was typically made at the state level. Without the patchwork of small, semi-independent states within the HRE, it seems only isolated populations such as would be found in the Swiss Confederation might be able to adopt Protestantism, if it even got off the ground at all.

What would the wars look like? Since there is also a centralized Italian state, maybe calling itself Romania or something, do alliances lead to World War type conflagrations across Europe centuries earlier? Does Germany come to dominate all of continental Europe, or do their neighbors succeed in keeping them in their place through allying with each other? Does Europe maybe get sick of wars led by Monarchs decades or even centuries before 1918? Does a European Union emerge, perhaps even before the English and Spanish Colonies in North America become independence-minded? Perhaps one democratic super-state emerges bringing peace and stability over all of Europe and the Americas in the wake of a decades long World War 1 during the 17th Century that results in mass peasant uprisings to overthrow the monarchies.