The Best Mod?

[Alexandr III. Biges]

-Only Rome should be able to build roads.

You know, Romans are famous for their paved roads, but others were building them as well, maybee not paved. But if you look at the map of Italy, it's mainly covered by hills (that's why Italy is famous for olives/oil and wine, not for grain production), so Romans needed good roads to move their armies fast and lately to cross mountains to reach their provinces.
Galia is more flat, so their inhabitants sin't need so good roads and with all likely nations they were not building empires. Eastern armies were far more cavalry based, so it's the likley situation and northen Africa civilizations more used sea and river boat transportation, but built roads when needed, but they usually didn't last to our times, because desert dand is far less merciful to human creations than snow of Alpes.

-Only civilized powers should be able to build seige equipment.

These "civilized" biult more siege equipment because they needed so so conquer their neighbours, but othe "barbarian" nations need so, they were quickly to copy them, they were in contact with greco-civilizations via traders.

-Playable Barbarian powers should be reduced to a single province with surrounding similar ethnic areas made rebel and much strenghened. This should make it much more difficult for any unification.

That would complicate the game too much, note that Romans and other nations as well were also divided and often wage wars upon their themselves :) When facing outside invasion, bunch of clans were usually able to unite to defend their land, that's the situation in RTW, which is war simulation, not trade or likely :)

-Elephants should kill and not just distrupt friendly units when stampeding.

When elephants move through friendly units, they take sare not to injure them, elephants are slow and calm animals, they are forced by their riders to attack furiously enemy soldiers.

-Barbarians should not be able to build fleets: save for perhaps the Britons.

"Barbarians" usually din't need fleets :) But they would be able to do so, if they would need. And this is reflected in RTW as it's "alternate history" game. And to the technology level of ships, northen nations simply built ships, shich were more suitable for their waters, trireme would do bad in Northen sea, I guest there were more storms and more winds, so ships there were smaller, faster and more wind and less oars depedable. Don't forget Viking later even visited northen America in likely ships :)
For example, do you know, that the design of majority of roman arms were inspired by gallic weapons? And their helmets too.
So I understand your opinions, maybee I'd even agree with some, would I know less 'bout the era. Anyway, it's good start for you to get to know more :)

Note: Sorry for mispells :)
Note: I'm not expert on the era, so anyone feel free to correct my answers :)