While I haven't done the whole historical victory I have 1700ad ish save where barring Britain I more or less kept the empire whole. Honestly its funny because I still have like 12 legions on standby to fix/build Roman roads. On the topic of ballistae I
often found that for there cost and role they are just flat out worse than training more legions. With the UP some cities can snowball real quick and churn out legions at a rate that you never really need it.
So here is my argument for a new roman UU
For a long time early Rome had an achillies heel, cavalry. The reason Hannibal was able to effectivly chew through whatever the Romans threw at him was because the romans really had no decent acess to or need for quality cavalry support. Romes traditional enemies up to that point had been the Greeks of Magna Grecia and other Italic tribes of the Italian penninsula. The mountainous and hilly geography made cavalry less effective and needed. Hannibals ace that the Romans had no counter to was his cavalry, his Numidian cavalry were decisive in that they routed the Roman cavalry and lured the Romans into the trap at Trebia, and his Iberian and gallic cavalry mercenaries are what sealed the crushing defeat at Cannae.
Zama ironically enough was the only battle of the Second Punic war where the Romans and Hannibal had parity in cavaly in terms of quality because a portion of the Numidian Massylii confederation defected to Rome and gave them the quality cavalry that ended up routing the Carthaginian cavalry and sealing Carthage's fate. From that point on up until the reforms of Augustus Roman armies basically sourced the vast majority of their cavalry from foreign mecenaries. By the point of Augustus all the practical sources of mecenaries were annexed and they were auxilliary troops.
My idea is to Nerf the Legion(not sure how) vs heavy cavalry somewhat since it wasn't the all powerful Swiss army knife that pop culture depics it as and make the second unique unit a cheap anti heavy cavalry cavalry, Equites Auxilia.(replaces horsemen starts with formation costs 45 hammers) The idea is its the support unit that covers the Legions weakness, while not being as ridiculously OP as the Legion is. This does raise the question why not just use spearmen? Historically the early Romans were not hiring them as a cavalry counter and that is why I emphasise cheap. They should be the superior(and quick) anti heavy cavalry choice to spearmen for Rome. Adding on to this maybe tweak the Celtic and Carthaginian AI and barbarian spawns to have more horsemen so they would be a more exisential threat to legions without an auxillay escort and early game Celts and Carthaginans a more potent threat until you can unlock the auxillia and legionaries can go in protected and conquer them. Being cheap also helps because with the nerf, the window of conquest would be pushed back since just loading your starting legions onto a ship and taking Carthage by 400 bc wouldnt be as easy as it is now.