A unique building would be nice and I am no fan of the liburna either but that is not the point I am trying to make. The Romans offer an opportunity for a unique play style if we can make it work with just their ability I do not mind the liburna. The tall Rome wide empire will make Rome a unique experience, two special units at the same time won't. I still think that roads should send production back to Rome not gold, gold might be better late game but Rome needs to be strongest in the early game.
Agreed, a flavorful Rome would be: tall Rome & wide empire. As stated, with the current UA (-25% production of any building already in the capitol), you'll be buying the buildings in Rome that you want to cheaply produce in your empire. The more cities you have, the more this UA is a benefit. (Moreover, I think it also applies to puppets, so even a happiness hungry conquering Rome still benefits.)
So, with a solid UU Legion that BUILDS roads and is available EARLY (however the resource requirements work out), then a UA or a UB (it could follow the concept of "all roads lead to Rome" with something like "Milliarium Aureum") that provides + and/or + when cities are connected to the capitol sounds much more flavorful and unique for Rome than adding (even a buffed & early) naval UU.