I agree that the free buildings UP smells cheaty. Maybe a less cheaty implementation of a similar concept would be the following:
*Add a new special specialist called Slave or Forced Worker, +4
for buildings and wonders, +2
, +1
*Whenever a Legionary conquers a city, half of the pop is removed, and for each pop one Roman city gets a Slave specialist. The cities are selected among cities with a Roman majority, from highest culture to lowest. If the script runs out of cities it will start again from the top (so if you only have three eligible cities and you just got 5 Slaves, the top two cities will get 2 Slaves). Each Slave only exists for five turns. (Alternatively, to require less checks, you can have one random Slave removed every three turns.)
*Whenever a city defender dies, all Slaves in that city die.
What you get is again a dynamic where your empire thrives as long as you keep conquering, making Rome have to keep conquering to maintain the large empire. In this respect it's realistic. It's not quite so realistic to represent the Empire's success as made possible by widespread slavery. A Power of Viae would be more realistic (and probably simpler.) But this idea is pretty damn cool.