And yes, these are legions with shovels. That was hilarious at first glance, didn't expect the game to be able to handle this without any input from me. The general AI also seems to work without any adjustments from my side - the legions seem to just have started building roads, when Rome is at peace, and only connect cities and don't pave the whole countryside.
I've seen Legions with shovels for a long time now.

I think it's a fitting image of the Romans' genius at military engineering. But you did it way better than I could. The Roman Roads looks like they're actually made of concrete. Very neat!
It is also nice that later European civs such as France and Spain can benefit from Roman Roads built by Rome. So that this feature is not merely a second UP of sorts for Rome only.
I haven't changed the UHV yet. I guess the building goal can be made more difficult (more buildings) or expanded by more building types (forums), but is otherwise appropriate.
Agreed. Forums should be included. Other building choices I can think of are Walls, Pagan Temple, and Courthouse.
I realize now that this UHV ties up nicely with the new UP, just like how the Four Great Inventions UHV ties up with the Chinese UP. That should make it more fun.
The third goal should be replaced either by a tech goal (I think Machinery, Civil Service and Theology are a good trio) or something related to Catholicism, although I can't think of a good goal at the moment.
I think a Christianity related goal is more suited for Byzantium. Especially with regards to interactions with Islam.
As for Rome, Theology's purpose is that it enables Theocracy (but so does Parthenon). The Roman Emperors (and their close relatives) were routinely deified, but as Roman gods instead of Christian Saints. That is until Constantine the Great of course, who is represented by Byzantium spawning with Theocracy.
*I think buffs depending on number of cities alone is too much prone to exploits in the end, therefore I'll try this first.
Yes, especially since Rome is one of the popular civs to go for Domination.
Although there may be exceptions. The Dutch UP goes stronger in proportion to the number of cities you have. The Russian/Muscovy UP in RFCE is explicitly related to the number of cities. Yet both civs are more or less balanced.
Thank you Leoreth! I foresee that the new Rome will become as popular as your other two masterpieces, England and China. May there be even more great works like this in the future!