I gueas the issue is that a Rome which builds Great Walls doesn't feel much like Rome... Cosmetic changes would be a step forward but a pretty small one I suspect.
Mods to let earlier era civs transcend to later ages already exist. There are some mechanics needed for that to work which would be good to have Firaxis' official stamp of approval on, but the fact that it can already be done makes me think that should be the mark for "minimum required for passing grade" rather than cosmetic changes.
Except Rome could build Great walls in Civ 1-5 and a Rome with a space program is equally unlike Rome.
I see 4 levels of “civ continuity” potentially available
1. Current.. you keep Traditions and already built uniques and city names
2. Labels.. can choose keep the name (and graphics and city list) of your civ but get new uniques for the new age (inspired by a separate real world civ)
Narrative: your people have changed how they do things in the new age, should they be called something different now?
Pros: Rome into Space, probably relatively easy to implement, no game mechanics changes needed..could also do American spearmen if you can choose your starting name…Could have leader match civ for whole game
Cons: some think American Legions or Roman Conquistadors sounds off. (would need UI to make it clear what bonuses AI is using if AI can be made to keep same name)
3. Semi-Generic bonuses..when Rome goes to Exploration of America is in Antiquity they get uniques based on their Attributes
Narrative: Rome changes but retains its Roman “Attributes”
Pros: Rome can retain some “Roman ness” in Modern without taking from France or Russia, not a lot of content to be added, Rome has modern age appropriate bonuses
Con: Still lose purely Roman uniques
4. Eternal uniques …Antiquity America builds str 25 Marines Modern Rome builds str 50 Legions, etc.
Pros: fully retain uniques
Cons: some uniques would be bugged or very imbalanced would require a lot of work.
I’d like to see 2 in the main game. and 3 as a mode (maybe in the main game if the Attribute uniques can be balanced) …4 is better as a mod as it doesn’t need to be as bug free or balanced.