I've wanted to design my own Eldar Craftworld for a while now, ever since the Eye of Terror campaign, actually. Nothing has really stopped me, but I find that my best ideas come from a germ of an idea that I simply let bang around in my head for a while. Being an old hand at world-building (I've been running D&D games for 20 years now), I've discovered that this can be a crucial process for weeding out bad ideas and developing good ones.
If you're not interested in fluff, you can probably skip this article. If you're a fluff monkey, you may get a little inspiration from this or at the very least some entertainment value.