The Wikipedia article will give you the basic rundown; some of the links at the end will tell you some of what happened. Besides not wanting to work with DC ever again, two of the big things that Moore had gotten out of working on Watchmen were a fascination with the way that connections between different elements in the book had almost seemed to make themselves (this was a bit before the Glycon phase, I think), and that he was more interested in the interactions between the people at that Manhattan streetcorner than he was with people who wore tights or walked on Mars.
Big Numbers (originally called The Mandelbrot Set, until BenoƮt Mandelbrot contacted Moore and asked him not to use his name commercially) was supposed to be an examination of human society as it behaved fractally. In one of the links from the Wikipedia article, Moore says that he still has the whole series plotted out on big sheets of paper, although he doesn't think it will ever see completion.