Although I have no objections to allowing wonders, I personally think they're silly.
If you can build it once, obviously you can do it again.
If I can build mine while you're still building yours, clearly I can do it after you finish yours too.
Artificial scarcity, pfft!
What I would enjoy seeing is a system where if you build it, you get an advantage. If someone else builds one too, then they cancel each other out until you go on a napalm crusade and reduce the number of them in the galaxy to just one again.
And of course, the ones you DO make would need to be logical. Not like Civ's Hoover Dam, for example.