I am always a fan of the slightly out of the norm, and unexpected. That's why, when I design bathrooms, I am always looking for something a bit out of the ordinary. One place I look for inspiration is fabric designs. The way fabric is made (with weft and warp) makes it a great resource for creating patterns for tile.
In the children's bathroom above, I used Gingham fabric as my source, when you study the design it is actually pretty easy to recreate in tile. A bit more complicated, but just as interesting would be a plaid design, or if you are a bit less, ambitious, a stripe would be great too.
I think it would be cool to use the real fabric as the shower curtain, and then recreate the same design in tile....now that will impress your friends!