Major tax and land reform in the early 2nd century to arrest the devolution of power to private estates and regional loyalties.

Something like the state deciding to farm the public lands themselves instead of granting this land to large private villas. The villa system led to a loss of the tax base (since in some regard the villa system was a tax minimization scheme) and also led to the development of the coloni class who were formally free citizens but through poverty became bonded to land and owing their primary loyalties to their local master.

This was the origins of feudalism and serfdom which was a devolution and regionalization of economic and political power in the centuries before the “collapse”. This was the weakening of state institutions that over several centuries resulted in the frayed condition that finally collapsed in the west.

So really it was the emperors allowing power to devolve to a more regional level.

I can easily conceive of an emperor in the early 2nd century after some civil war and economic crisis, he could look back to the deified Julius Caesar for inspiration, and become a populist. Advocating much like Caesar did for the people to get the use of the public lands instead of only the exclusive elite at the top holding almost all the rights to it.

So a populist emperor who joins with the plebians and perhaps also in league with some provincial groups against the senate. Let’s say this emperor wants to crush the power that the senatorial and equestrian classes still had and this emperor decides to do it by forming an alliance with the lower classes, presenting himself as a populist promising land reform to seal the bargain.

A redistribution of land creating an empire of mostly smaller land holders would have been a much more economically vibrant one than the actual Roman economy of the later centuries, the villa system being one of each economic unit seeking autarky to the maximum extent possible, mostly to avoid taxes. This desire to escape taxes by minimizing trade meant instead of shipping wine in from Hispania you’d much prefer to make your own. Trade collapsed and so did the tax base.

If instead there was a reformed economy. If instead of reducing the rural poor to the semi-enslaved status of coloni (the prototype for serfdom), if instead of this act of class warfare from above the emperor had allied with their cause in a bargain to cement his personal power from any threat from tve senatorial class there was an elimination of the traditional patrician economic elites who had a stranglehold on the use of land and the creation, in its place, of an economy of mostly small scale farmers as the most common economic unit, you would have seen a massive trade resurgence and you would have exposed most of the economy to the tax system once again.

Thus with an energetic trade network being the backbone of the empire you would have retained the infrastructure required for large scale trade, and therefore retained the capability of more easily moving forces around the empire.

You would have had far more money in the public purse by removing the patricians who were a tiny number of the empire but who enjoyed for their own private consumption a pretty staggering share of the wealth, and their ability to organize their personal wealth into closed economic units, villas, was the economic process that underlies everything else that happened.

It wouldn’t have been too out of character since Julius Caesar was quite a populist too and it’s specifically this use of land that probably got him killed. When he got to limiting how much of the public land any one individual could farm for themselves that’s really when the senatorial class he had previously subdued rose up against his dictatorial rule again. So it’s actually believable another emperor could have taken inspiration from this to form a populist alliance of the same nature as had Caesar himself.