I was recently stunned to learn that the Great Wall of China is not as ancient as I once thought; in its modern form it only dates to the mid-1500's, with its earliest brick-and-mortar section (in the Ordos Loop region) completed in 1474. While an earlier 'Great Wall' did exist, they were often a string of mainly earthen walls, made of dirt and gravel. What 'real' fortifications existed were not so much a 'Great Wall' as they were a string of border forts or walled cities along the northern frontier.
I wonder if it might be a good idea to alter the effects of the Great Wall wonder in-game to better reflect this reality. The two possibilities would be to either move the tech prerequisite to much later in the game (to better reflect its modern construction) or to keep its current tech prerequisite and alter its immediate effects. This second one would be my preference -- I'm a big fan of playing as China, but I'm inclined to say the Great Wall is fairly overpowered, at least in the basic version 1.12.
Given its historical disrepair, it's hard to argue that the Great Wall did succeed in keeping all 'barbarians' out, though it had some use against invaders from the eastern steppes. I'd be inclined to add it as a combat modifier, maybe a generic +50% or more against all barbarian units. Or use it in conjunction with the Terracotta Army, and double the number of experience points (and hence Great General points) gained in combat with barbarians.
One of the Great Wall's more interesting (real-world) effects was when it was extended to the Jade Gate (Yumen Pass) to protect Chinese merchants travelling the Silk Road. I've never seen a proposal to use it in that context, as a boon to commerce rather than (or in addition) to defense. Perhaps it could function much like the Great Lighthouse by adding trade routes, or by permitting trade routes to form between you and independent cities, or by adding a bonus to existing land-based routes, or adding a bonus to the 'Silk Road' corporation, or even by allowing the Silk Road to expand into your territory in mainland China.