Big difference of Roman and China is how it is ruled. China is one block, not relying on a Qin culture. Rome is central state administrating colonies. For a long time they refused to grant citizenship to territory. When the begrudging did, they made little attempt to assimilate. The locals might try to identify as Roman spontaneously, but only in their own interpretation. Their 'Roman' identity is different from another region's "we are Romans too".
Greeks think they are Romans due to being having cemtral government, but they are more or less secular, not obeyimg the pope.
Germans had close relation with the religious figure and own the city itself, but speak another language, never part of previous Roman state. They are quite convinced they are Roman but had little acceptance in other region think they are Romans.
So while they all think they are Romans, they had little in common, and was hardly fully Roman in first place. There is no reason to accept reunofication. From standpoint of Goth Romans, being ruled by Greeks is more or less ruled by a foriegner, despite both sides believe in the Roman idea. When Greeks see Germans, they see aliens taking over their land, ought to be eliminated. When Chinese unify, they see the conquorer as one of themselves taking mandate of heaven.
So the lesson here is having your regions believe central legitmacy is good but insufficient. Between regions they must also harmonize, see others as the same family. And if we are all in same family, whats the difference between a lord from Nanjing or Beijing? Only then do people accept unification.
We see similar idea as Chinese in Europe 19th century. Between all us Italians, who cares if Italians from Rome form the kingdom or Italian from Sardinia? I believe in big family look after each other. I have faith Sardinian government will treat us as fairly as local lord in Milan. Only thing between us are the lords squabbling, and we can get rid of these pesky lords for all we care if it means good for the family. As we see, Italy unified. Rome never did.
You could see Italy unify from a never existing country, because regions harmonize. Rome had such prestige and history, yet could never unify because people in each regions are never properly harmonized.