Units I have in mind:
Aztecs:
Teopixqui (Monk replacement)
Bengalis:
Brahmin (Monk upgrade when Paiks researched)
Britons:
Billman (Halberdier replacement)
Bulgarians:
Volga Rider (Secondary UU line available in Stable starting in Castle Age)
Khmer:
Royal Battle Elephant (Elite BE replacement)
Byzantines:
Variangian guard (Champion replacement)””
Celts:
Gallowglass (Mercenary unit available in III age)
Chinese:
Fire Lancer (available in Range in III Age, weaker than HC)
Cumans:
Pecheneg Warrior (available in Castle Age stable)
Burmese:
Culverin Elephant (Available on IV age)
Ethiopians:
Kushite Archer (Arbalester replacement)
Franks:
Gendarme (Heavy Cavalry available in IV age)
Goths:
Vandal (Champion replacement)
Huns:
Avar Raider (Secondary UU line available in Stable starting in Castle Age)
Japanese:
Ashigaru Arquebusier (Hand Cannoneer replacement)
Koreans:
Hwacha (Siege Onager replacement)
Magyars:
Corvinian Knight (replaces Cavalier in IV)
Malay:
Djong (warship)
Malians:
Tuareg (Camel Hand Cannoneer)
Mongols:
Turkoman Raider (CA replacement in Castle Age)
Mayans:
Eagle Slinger (Available in II age Archery Range, reference to Eagle Balamob Slingers in Rise of Nations)
Saracens
Imam (Unique Monk)
Sicilians
Norman Knight (Replaces Knight in III)
Slavs:
Cossack (Light Cavalry)
Teutons:
Crusader Knight (Available in Castle like in the Dawn of Dukes campaigns)
Turks:
Akinci (HCA replacement)
Current civilizations with 2 unique units are omitted.
Do you have any suggestions? Share below!