Well I can tell you a (rather stupid) idea I had some weeks ago: sort of an external device alarm.
I say it's stupid because it'd be a bit annoying, but rather than it's usefulness I was interested in the fact that it could be easily done.
Anyway, it'd go like this: say you brough your laptop to some customer's office or someone else's house, and aren't all that confident about your stuff (phone, pendrives, mouse, the whole laptop) not disappearing as soon you walk away.
What if you could set it up so whenever anything is disconnected (usb stuff and even the power plug) it activated some action like ringing an alarm, taking a picture with the webcam or maybe sending an email with the event and the pictures?
For usb devices you can set up udev events on plug and unplug, and then use some graphical frontend to let the user decide which devices will raise the alert.
For the power plug IIRC you can check that on /proc
You would also have to rise the volume when the alarm fires, and I found back then an article somewhere about a way to override the mic and phones plugs' settings, so you could disable the phones plug so it goes through the speakers no matter what.
As I said it might not be all that usefull anyway, but it's easy enough to keep you entertained for a while without much headache, if you feel like it the different pieces can be done in a matter of minutes, the actual work is putting it all together and packaging it nicely.