JR,
I don't think it's technically proper to call the Later (Western) Empire and the Byzantine Empire "theocratic." In the latter case, the State controlled the Church, not the other way around.
The Emperor, at least in pre-Dominate Roman Empire, was the Pontifex Maximus, the head of the Roman state religion. But
afterwards, and especially after Justinian, it took on a more directly theocratic tone. The Church was subservient to the Emperor because the emperor was the head of the Church. He was God's annointed representative on Earth, sent to protect, rule, and lead the Universal State that was his obvious intention for Man.
And didn't the "military dictatorship" keep the Senatorial class under its thumb? It's true that a lot of the dictators were from the Senatorial class, but I was under the impression the Senate was under control.
Yes and no. The politics of the Republic remained long after the spirit was gone. To be an Emperor secure in his position, you needed three groups on your side: The army, the Senate, and the People of Rome. The army was relatively easy (very relatively): Lavish them with attention and victories and they didn't care. The People were easy, Bread and Circuses were all they cared about. It's the Senate was the difficult proposition (again, up until about Justinian). It was often army officers of Senatorial rank who would be the main challengers to the security of the Emperor's throne.
Here's a thought. In Byzantium, the Emperors supported freeholding peasants to weaken the aristocracy. What got that going and how might it be replicated in the West in a much earlier timeframe?
You would need to maintain security in the West, for one. Whereas, in the East, the 'moral' government of the new Christian regime slowly helped bring about a change in land distribution (among -many- other causes), people clustered around manors in the West for safety and security in very uncertain times. Whereas the East managed to maintain something approaching security through the crisis of the fifth century, and managed to eject the Germans from the halls of government, the West saw its borders collapse and 'barbarian' tribes (who were as often Romans turned criminal from desperation as they were Germans or Huns) roaming at will, virtually free to take whatever they could pillage. Likewise, they failed to force the Germans out of their government, so the Western state quickly turned into a puppet for German warlords.
One of the major causes of this was the sheer length of the Western border versus the length of the Eastern one. The East's longest border was with the Persians who were, if not entirely friendly, at least another civilized power, not given to occasional mass migration. The West, however, looked over the Rhine into what was basically wilderness. When the Rhine froze over, there was nothing but a thin red line of Roman Legions facing the Germanic tribes.
Pushing out towards further European rivers will help shorten the border, but it might also help preclude the conditions that brought about the 'equalization' of Roman society. The East only really attained a 'real' class of yeomanry
after the Avaro-Slavic and Arab invasions, when the entire political and economic structure collapsed. Faced with the cut-off of Egyptian and North African grain there were suddenly a great many urban mouths to feed and a great deal of newly virginized land to be reconquered.
One of the main reasons China managed the system it did was because it makes sense to settle soldiers on the frontier: You've got a ready set of already trained fighters right where you'll probably need them most. The problem with Byzantium was they ran out of unsettled, prime farming land to expand into. By the time they hit their apex, everywhere else around them was already taken.
Perhaps some Emperor rewards his veterans with lots of lands from Senators he dislikes and makes sure it stays that way?
Well, the thing is this happened IOTL. Roman soldiers, upon retirement, would be granted land to farm. The problem is that it was mostly land in the provinces: You had to be REALLY important to get land in Italy. Unfortunately, it was the land in Italy that mattered, politically speaking. The provinces had no representation at all in the central government. Since the aristocracy held almost all the land in Italy, they de facto controlled the government. Angering a bunch of Senators in Rome was a much more serious threat to an Emperor than angering some soldiers along in Gaul (at least under the Principate...once that fell apart, it was actually the other way around).
Furthermore, I was under the impression China had a gentry class. The "free peasants subject only to the Emperor" seems a bit idealistic.
They did, but they were emasculated and their influence was actively fought against by the Emperor. The peasants were mindful of this and believed their main loyalty was to the Emperor. The strong urban middle class made this connection even more solid, by giving an additional base of support free of artistocratic influence.
Rome's problems were, basically:
A. The main agricultural settlements around the major urban centers were plantations worked by slaves.
B. The Crisis of the Third Century effectively destroyed what urban middle class had existed in the West. It survived in the East only because it was more entrenched there. The West had always had a problematic relationship with its urban proletariat. The original 'Sesession of the Plebe' events that shaped the Early Republic were mainly caused by the outright hostility the Senatorial class showed towards any kind of trade or urban production at all.
So I maintain the best PoD to get a Rome that survives and progresses would be to go as far back as the Gracchi. Their land reform attempts were the last, desperate gasp of the reformist spirit that had made the Republic an institution worth saving, rather than just a power collusion amongst the nobility after they threw out their Etruscan king.