The Raspberry Pi contains a lot of interesting bits that we aren't yet doing anything with. Here's a (more or less complete) list of things we'd like to see done, with some info on each.
The best starting point for an overview of the Pi's hardware capabilities is on their wiki.
Bakul Shah has been working on a GPIO driver for the Pi.
These are display and camera interfaces, located at S2 and S5 on the Pi, respectively. These are not widely supported on the various systems which run on the Pi, and compatible hardware is not very common. Compatible displays can be found in some smartphones.