Well, I can think think of a few. Movable type is one. The Romans could be pretty inventive when they wanted to be and were fairly literate by ancient standards. It does not seem at all out of the question for the Romans to have invented the printing press. Except they didn't. Another one is an Imperial canal system. The Romans were master engineers. They concieved of and built an Empire-wide system of roads for moving Legions around and to facilitate trade. They also built a network of aquaducts to transport water. Canals are a logical extension of that and a perfect fit for the Romans' engineering skills, capabilties and inclinations. Plus they keep slaves, so they have a huge pool of free labour, so they could have dug a series of Imperial canals reasonably cheaply. The third one is the bicycle. We've discussed the possibility of Roman bicycles on the board more than once and the consensus is that the Romans should have been able to build them, and they were sufficiently skilled at road-building that they would have have places to ride them. Bicycles would have improved communication between cities and given more mobility to the lower classes. These are all ASB in that Romans just didn't think of them, but are all well within the Romans' capabilities.