I've had a fair bit of experience with modding and modding communities, particularily with the Baldur's Gate 2 modding communities. One of the issues that seem to crop of frequently when people want to do something is "Well, we'd like to do X, but its hardcoded". Much of the higher level modding ends up involving lots of little tricks to get around hardcoded features. In order to make Galactic Civilizations more mod friendly and to extend its lifespan accordingly, I have a few suggestions.

1. Try to avoid putting hardcaps on the number of slots for things. Number of slots in ship design, number of slots in technologies, number of worlds in the galaxy, etc. To give a Baldur's Gate 2 example, one of the things that people ran up against was the hardcap on the number of creature animiation slots they could add. There was room for about 40 more creatures, but after that you had to start replacing things, or trying some fancy coding. Since I haven't had a chance to take a look under the hood for the code, I can't give exact details on what things you don't want a hard cap to be placed on, but as a general rule, if you can avoid it don't put hardcaps on things like that (and I don't consider 65536 to be a hardcap when you are normally dealing in the 50-500 range for number of slots being used)

2. Allow the campaign to be changed/modified. After people fiddle with ship designs, this is the next obvious place for people to tinker with. I can already forsee Star Wars and Babylon 5 mods being made that could make good use of a custom campaign.

Anyways, thats just a few ideas to start. Hopefully other players can add more things to the list of what they'd like to be able to tinker with. Maybe once I get my hands on a copy I'll be able to add to this list.