In my opinion, I would try to make a huge dark mages tower, made up of mostly dark materials. I will leave that up to you which materials specifically. (If you are having trouble choosing, go to this thread. It will help you with color choices. http://forum.feed-the-beast.com/threads/this-old-9x9-challange-and-share.25358/ ) Also, you probably want a whiteish high-contrast color for part of the inside, again, I'm leaving you to decide. I would try to light it up with nitor. If you will also be doing non-Thaumcraft stuff in it try to hide most of your technology (IC2 and Buildcraft machines) and make sure everything exposed fits the theme.