AD or BC?

And why are you limiting it to such a specific time frame?

There is something called the Marian Reforms, which are considered to have been immensely important and consequential, but they were carried out in the late 2nd century BC, in response to wealth inequality during. .. well, prior to the 1st century BC, obviously.

Edit. to add a bit more: around the 2nd century BC Rome was facing a manpower shortage, as wealth inequality had reduced more and more independent small scale Roman farmers (the demographic the Roman state had drawn their infantry from - people wealthy enough to supply their own arms & equipment - under their traditional military system, a citizen militia similar to that of Greek poleis' systems) to the ranks of the poor. To solve this manpower problem, Marius extended military recruitment to the poor, with the state to supply the necessary arms. At the same time, he changed the Roman legion from a part time militia called up for service for the duration of a conflict or military campaign, to a full time professional force offering the prospect of long term employment.

Other than the predictable immediate effects on military effectiveness, this had tremendous long term political consequences. It enabled Roman generals to become basically something akin to warlords, able to command the personal loyalty of the legions under their command. Think e.g. the Triumvirs of the 1st century BC. Julius Caesar, Pompey, Crassus, Octavian, Mark Anthony, Lepidus. It thus enabled the civil wars & power struggles of the 1st century BC, leading to the fall of the Republic, and its replacement by the Principate.