Okay, I've finally gotten around to do something about this. I've made the following changes so far:
- significantly increased Roman unit production
- reduced Legion cost from 45 to 40 (same as Swordsmen)
- removed the old UP, gave AI only conquerors instead
- new UP is the Civ5 Roman feature: +30% building construction for every building in your capital*
- gave Legions the ability to build Roman roads, who are like usual roads but give +1 movement with Masonry instead of Engineering, so they will be superior to these until Engineering becomes widespread
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.
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 buffs depending on number of cities alone is too much prone to exploits in the end, therefore I'll try this first.