If gunpowder had been known, IIRC you could get a Sten gun with Roman metallurgy.
But as RamscoopRaider says, the inefficiency of agriculture in man-hours to produce the food to feed one person is a biggie. In particular, the reliance on either slave labour (cf. Sicilian slave rebellions) and imported food (from Egypt and North Africa) mandates against seeking to vastly improve Roman agriculture through automation with draft animals, seed drills, crop rotation and the like, as it would devalue the asset holdings (i.e. field slaves) of the wealthy and the good.