Good way to add unique units to Return of Rome civilizations would be if each faction would get unique unit upgrade instead normal iron age final upgrade. All those unique version of these units could be little bit better or have some sort of special abilities compare to regular units.
For example:
Persia would get Immortal instead Legionary
Macedon would get Companion cavalry instead cataphract or Silver Shield instead Centurion
Egypt would get Iron age upgrade to Chariot archer
Carthage could get African Forest Elephant instead armored elephant/elephant archer or sacred band instead centurion
Minoan would get Cretan Archer iron age upgrade to composite bowmen
Hittites could get Three man chariot instead scythe chariot.
Etc…
This kind of approach would mean that game wouldn’t need any special building for that.