I have a few ideas for possible build contests.
1. Recreate a famous landmark building. Basically, take a building like the White House, and try to create it in the sims. We could post pictures of the real building for comparasion. For the inside, you could do realistic, or totally revamp it.
2. Build a doll house, like the open face houses that little kids sit in front of to play with their dolls.
3. Build something out of storybooks, such as Cinderella's castle, the 7 dwarves cottage, the three bears cottage, etc.