The problem with such an extension is that it would require a lot of adaptation.
Perhaps one day in the distant future the game could extend itself so people could play underwater, or attack alien bases (instead of "only" defending earth), but it sounds as if you are suggesting things which quite like take years of developers time away too quickly, and I am not sure that this would help finish some other issues at hand at all ...but here is another idea, we could try to recruit coders from china solely working on these ideas, to make some kind of sequel to UFO:AI