App that is the modern equivalent of a journal. Users have different templates for different journals. The could be differentaition between public journals (more like a blog), private journals and protected journals (journals not open to the public but can be shared between friends).
I remember as a kid having a journal. I wouldn't write too much in it, but I think that a kid-friendly journal app could be cool. There are several ways to accomplish this coolness factor. Although, I personally wouldn't limit my potential users to only kids, this is an interesting idea.