There should be a wonder you can build once you discover currency that offers 5 culture, never expires and the effect of it is it gives you a marketplace in every city on the same contenent. It would probably be quite expensive.

What do you think of this Idea?

Put it in your game.

Pick a wonder first, then pick it's effects.
Personally, I like the idea of a wonder for each city-improvment.

A marketplace in every city that never expires is pretty powerful. I'm all for stronger wonders, but remember that Marketplaces can give about five extra happy faces, given the right conditions. Of course, if you knew somebody had the wonder, you might be inclide to withhold your luxuries from them.