Conquest, on the other hand, I think is feasible. Why not include a very small chance that a wonder will be destroyed on capture, much like normal buildings can be? Of course, certain wonders should be exempt such as the Great Wall, maybe the pyramids, etc. but more conventional wonders like the Great Library or the Parthenon would be (and have been) completely susceptible to collateral damage. I think the "legendary status" idea upon losing a wonder is a little too abstract, but maybe if a wonder were destroyed on capture, ruins would be left that would retain the cultural advantage and lose all the others?