Since the Great Wall is now exclusively Confucianistic - have you considered the
Limes/Hadrian's Wall as a knockoff for the classical west? (Edit:
Of course you have. So I promise to not present a simple copy that would make it not-unique)
Having played a bit as Rome now, a little reward for barbarian-squashing is much needed: You get some generals and experience against gauls and rival civs, but the later hordes are really a chore.
Regarding requirements: Construction, not built in core/historical area, and also at least 6 (8?) non-core cities. Similar expansion doesn't exist for most other classical empires, except for Greece, who is also an acceptable candidate to build it imho.
Regarding costs and production bonus: It should be pretty expensive without bonus - and stone shouldn't grant a bonus, seeing that the Limes was woodworks, earthworks and landscape sculpting in most parts.
But each wall in a non-core city could give a +25% production modifier, as should forts if any exist within borders. This represents how the defenses of the Roman Empire all worked together to form a multi-layered defense. Or possibly, the Limes could be erected automatically once the 8th (10th?) non-core wall has been built, so that the player doesn't need to actively build the wonder.
Regarding effect: A culture bonus of only 1, and a similarly small great engineer bonus. If not zero. I can think of several alternatives for the main effect, but the economic one is my favorite:
- military: enable regular Great General points for defeating barbarians (would lead to a high number of generals though).
- military: allow the owner's military to gain experience up to 17 XP points in fights against barbs, thus a higher XP cap - I had a few dozen legions around who were all at 10 XP...
- military: +50% against barbarians and +25% against other enemies, inside cultural borders
All of these possible military bonuses however lessen the pressure that should lead to Rome's downfall. So instead I propose
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economic: awarding some wealth per defeated barbarian/enemy, inside cultural borders (I'm thinking about slavery, but it's depending (on civic?): could be wealth, production, science, culture, great statemen points, espionage?). The task to defeat all the barbs has still the same difficulty.
Regarding the art:
Not a Limes all around the borders. A single Limes tower in one city should be enough as a representation.
Finally an early expiration date: Feudalism and Fortification seem to be obvious obsoletion techs - so the medieval civs can't profit at all.
Of course, my game experience comes from 1.15 again, but I have given the Aqua Appia some thought: Except maybe for Rome, I didn't encounter health problems in 1.15, seeing that they already get aqueducts and baths. Shouldn't the effect be "free baths in all cities" instead of "+2 health" on top? Just my 2 halfpence.