I originally posted this in the mods section of the GC2 forum, but that section doesn't carry over to the Impulse forums. I'm re-posting on the Impulse forums since that's where most people are going now, including myself.
I put up a Twilight mod I did for my own use. It's pretty extensive and makes the game quite a bit different. It doesn't change any graphics, just game characteristics. The biggest change I made was to rework all the tech trees. There's tons of other changes as well. Look at the INFO.txt file in the zip archive for more information. Also look at the README.txt file. Here's the link;