I'm planning on making a koi and dragonfly quilt for my bathroom. It is from the Foundation Pieced Nature Quilts book. Sadly it has gotten pushed down to a lower priority to make way for some other projects. I'll get to it next year though. I'm planning on making quilts for all the rooms in my new house. I'm going to do frogs in one although I'm not sure which room that will be. My boys will both have their own bathroom so I'll let them pick what they want in there. I bet my youngest son will want a Knight theme :D
For your bathroom, you should pick a quilt that you would want to look at whenever you take a bath... something that makes you happy. We could suggest things however you'll be the one looking at it every day.
As far as the wall color I'm sure that it will go with whatever fabric you pick. It sounds neutral enough.