If multiplayer ever gets considered for one of the expansions, I'd suggest looking at a Pitboss-like multiplayer type. Personally I'd prefer this many times over to i.e. online / LAN play.

Pitboss refers to a multiplayer type in which (in general) the players log on to a server ran by one of the players in the game. The server player can specify a time limit in which the turns have to be made. If someone hasn't made a turn in time, his turn is automatically skipped (or the AI plays his turn for him). The main advantage compared to PBEM is that games go alot faster (and i.e. turns don't get 'stuck').

Civ IV i.e. has a Pitboss facility although it's pretty buggy.

Note: I also posted this suggestion in another thread, but that one is 3xx posts long, so I figured I might as well also post it here .