Trade cities is a variant we used quite a bit back when I played this regularly. It gave strategic importance to maintaining certain cities, rather than simply the number of cities you owned. We also regularly used the Imperial civilization variant (2 sets of tokens per player), as well as the Atlantis variant for Crete.
Sometimes we used all 3 at once...!

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I should also say that i'd be very interested in resurrecting the trade city variant, using all the commodities from the new, expanded 18 player game.

Chern