On a recent project with Wamhoff Development, our client had this thought too! They really let their personality show through and created a space that not only showed who they are but amplified the rustic theme of their home.
Now, this is just ONE idea. You can do anything you want in a powder bath. It is a small space, so splurging a little on tile or wall paper wont break the bank. And remember to have fun and make it your own.