I realize this is an old thread, but I came across it whilst doing some research.

I am interested in using GIS to build a fantasy world as well. The reason, honestly, is because I want learn GIS for professional reasons, and I know that I'll have more fun with the learning process if I can use my [over-active] imagination.

Is it possible to start small, perhaps with a map of an island, and expand outward from there. How much base data does GIS need? Can't I just create the island and fill it with various data?