I know there has been strong arguments against it and I would agree that multiplayer would be pretty boring if was to be based on the current turnbased set-up. So what I would propose would be to develop and sell multiplayer as an expansion pack and make it very different from the singleplayer turnbased play. My idea would be to turn it into something similar to the classics VGA-Planets and Stars!, which I am sure there must be other people than me who remember.
The basic idea is this:
Every turn all players lay their plans and then send a small file with their commands to a game host. Then the game host calculates the actual outcome of all those plans and sends back the infomation each player needs to see what the world looks like now. The host could be a volunteer or even better it could be run by Stardock sort of like the Metaverse is.
As part of the setup the speed of the game could be set to whatever people agree on, so there would be fastpaced games played live with turns being run maybe every 15 minutes. And there could be slower games where you just have to submit moves once per day. The host could be set to only process turns at fixed intervals or it could be set to process them early if all players have submitted. The latter option would mean the game speeds up if all the players happen to submit their turns early, but that it will wait if some players dont have the time to do it.
It would require some changes in the game engine since the player wouldnt be able to respond to pop-ups during each turn, so some things would be designated ahead of time. For example, like what modules a starbase should build when a construtor reaches it or what tactics to use for planetary invasions.
There would be a set order in which all the different things you can do are executed. For example planetary invasions would propably happen before ship production so that your invasion against an empty planet cant be foiled by building 1 ultra-cheap ship every turn that would then have to be eliminated.
Apart from giving the existing fans of GalCiv something more, i think this would draw in a whole new crowd of players to buy the game. I know that a lot of people are disapointed that Stars! Supernova was cancelled/died before being finished. I know that I would love getting to play GalCiv2 like that!
