We'd probably start with Tribal troops but then the next level of barracks builds Hoplite style.
Then we'd move on to Polybian troops, like the same kind of development of Roma Surrectum 2 at the start.
After that would come Marian straight away, Augustan after next barracks reform, allowing to play until the start of Lorica Segmenta mass use.
So then, we get Hamata units straight away, with imperial weapons etc. We see Barracks upgrades and various other things which allow Segmenta units which have significantly lower training cost and slightly improved armour (hamata was more or less as strong as segmenta, but just harder to produce).
After that goes on for several hundred years (the height of Rome's power), we'll make Legionary units more or less the same but smaller unit size, and more expensive. The player will be more or less forced to use mainly Auxilia + Foreign mercenaries.
The fourth and final reform will see the rise of troops Commitatensis style
Thanks for the quick reply, it's great to have an experienced guy helping design