Thaumcraft gives you some nice toys to play around with. Some are for creative and fun builds, like the arcane levitator, the arcane ear, the warded blocks. The arcane ear is fairly interesting in that you can use it as an alternative to Wireless Redstone, with a more limited reach. It can also be used to password protect anything that responds to a redstone signal, since you tune it to a frequency only you know. The Arcane Levitator is a pretty fun way of going up and down. Warded blocks are pretty, but they're also very secure. They allow things like, for example, a Nuke cannon, or a directed nuke blast. The combat golems are a pretty useful defense system, along with vanilla iron golems and snow golems. Some pretty nifty fights can be staged by combining it with soulshards. If you have the Vis to spare, the Arcane Bore can be a pretty interesting laser display show, just have to combine it with some way of automatically replacing blocks as fast as they are mined so it just keeps working for a good while. The Infernal Furnace is useful for triple meat treats. The Hungry Chest is just fun all around, but very interesting as a trash disposal when connected with void pipes.
As a challenge, automating thaumcraft ingot transmutation is pretty hard, so there's that. Also, thaumcraft crafting in general, is almost impossible to automate effectively, so, another challenge, as is automating aspect production and storage.
And i'm all out of ideas for now...