My two cents...
I voted for the camoflauge, thought that might be interesting in different colors, say for example green, pink, blue and purple. I also liked the ideas of solids and translucent as well as animal prints. One could have leapard spots or tiger stripes.
If you can make it change colors, what if you can make it to turn red when its in danger of dying, sort of an extra warning system in case the sound is turned off.
I also liked some of the ideas others have suggested above and I do not envy the task of having to go through all these. I'm sure whatever you guys pick, it will look good.
honerable mention, create your own. That would be interesting. Perhaps it could accept skins somehow like ipods do or laptops. Then you could sell the skins online and on stores. Also the idea to draw your own is appealing. That would require a cover that would accept paint and then perhaps a coating to protect the person's work once the art is completed. So many wonderful ideas....