TOo funny, I just turned in an upcoming article about bold wallpaper in the powder room a few minutes ago. Powders with powerful papers are fabulous - go for it! The door will be closed, so it doesn't need to flow with the rest of your home if you want to do something wild, and it's a fun surprise to anyone who opens the door for the first time. I think a medium to large scale pattern could look smashing in here and wouldn't compete with the smaller pattern of your countertop. Because your vanity is to the side, I'd say go from baseboard to ceiling with it.