OK, let me back up from what I just posted, because obviously I'm being hypocritical.
Upon reflection I agree that what you decide to do may diverge from what is legally allowed, as just for full disclosure my current project draws heavily from what is certainly copyrighted material. Some of it I might claim "fair use," but a lot of it is just out-and-out "taken" from another source, to whit, SMAC.
Now I'm aware that at any minute the publishers of SMAC could throw an injunction on us and halt the project... which is one reason why we are intentionally building the mod to be a "remix" of the concepts
but not actual content of SMAC. We are using SMAC images, etc. as placeholders until we get to that point, but I will be content to drop all of those if need be, and in fact I would see that as desirable as I would like the mod to have its own existence.
However, there is another team over at Apolyton which is working to faithfully reproduce SMAC, and while I think they would be in a much stickier situation if the publisher went after them, they have already contact said parties and are negotiating a resolution. Further, Civ4 and SMAC share a common studio (but not publisher), and both products have been modded (the former even offers support for so doing) in ways that indicate the publisher tolerates this activity, which would help a fair use argument.
Given that SMAC is for all intents and purposes a "dead" product, and that Firaxis repeatedly fends off efforts to pin them down on a SMAC II, there could be a "moral" for why letting the community create SMAC II on their own is OK. But I would not bet the farm on that effort's legality.
Every single mod here that is based on a franchise (e.g. Star Wars, Star Trek, Ringworld, whatever) is treading on thin legal ice, but the practical consequences of doing so are usually quite different than legal rights.