It'd be nice to be able to fall asleep listening to some music and not have to recharge my iPod for the next morning. A sleep timer wouldn't be TOO hard to implement (in fact, it could probably be done with a patch) and it would save the battery life.
Configurable times, like 90m down to 15m or so, and of course the ability to turn it off in case you don't want the sleep timer on anymore. Also, a way to see the remaining time.