I do think Firaxis could/should tweak/add uniques to civs to help them out of age. For example, France could get a "gaul" unique building in the Antiquity Age.
I think that would be impractical.... because it would mean essentially creating 86 new civs (as there are 43 currently in the game/coming DLC)
This is why I think having it based on attributes might work better. So in Antiquity
France would get the Diplomatic and Militaristic UIs, Traditions and bonuses
Normans would also get the Diplomatic and Militaristic UIs, Traditions, and bonuses
Mongols would also get the Militaristic Bonuses... but not the Diplomatic ones they would get the Expansionist ones instead.
That would still mean creating 18 new partial "civs" (but they wouldn't have their own graphics, art, music assets, and wouldn't need as detailed historical look)
Essentially each one would have ~2-3 civics with 1-2 traditions, . give some bonuses to military or civilian units (as opposed to UUs)... and you could get a choice for which one gave you its UI.
Another possibility would be to have it like Modern Ideology... Antiquity France can research the Diplomatic Antiquity Tree or the Military Antiquity Tree but not both.
(it would still get the standard Diplomatic & Military Attribute points at the standard times.)
That might be the best... because that way You can choose what You want Modern Rome / Antiquity America / Exploration Tonga to be... to you want to make your Assyrian Empire a Scientific Powerhouse or a Military one... or perhaps Scientific in Exploration/ Military in Modern.