I know it's too late for suggestions, but I had an idea...
When a city is destroyed, it leaves behind ruins. If the ruins are in a cities' radius (or empire's borders), they could be used for tourism after the discovery of, say, capitalism.
If the destroyed city had wonders in it, they increase the tourism revenues.
I had once a thread about tourism (and I guess some others had some too), but I didn't think about that way of tourism, but it could be intersting. But then it could be like if the civ (who build that city) is still alive the tourism income is not as high as if that civ is gone. And the income could also depend on how long time ago that city was crushed: The longer time it has been abondomed = More tourism income
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