Here are a few ideas:
Make a huge castle out of stone bricks with lots of lava features (worked for my server)
Lapis Lazuli and Iron Blocks look great together so you could try to make a building out of them.
You could make a gloomy dungeon type place with water dripping through the ceiling.
You could make a pyramid out of different types of sandstone (lava looks great in these two :D)
Lets you ideas expand but feel free to use any of these suggestions :P
~miklvy