Well, I've been using Rockbox for some time (thanks to God you finally managed to put it on the Nano 2G), and I think I've a good idea for it.
Some time ago, I discovered something called Binaural Beats. Perhaps you know them with the name of Digital Drugs, thanks to iDoser.com, although that's not their main function. They're sound which can alter the conscious state, very useful for meditation, astral projection, painkillers, and some weird and interesting stuff.
I write this because of Sbagen, which is a FREE binaural tones generator (not like that from idoser), which has been recently ported to Android.http://www.normalesup.org/~george/comp/binaural_player/
Using the same modified code, the creator of the Android version told us that could be very easily developed in Rockbox. That would mean, as the doses can be more than half an hour long, using the standard .sbg files, which are only a few kilobytes big.
So, I would be very pleased to see my Rockbox player using my Binaurals, and so would most of the Sbagen community, i think.
The home of the project is http://uazu.net/sbagen/
, and the programme itself runs on command line mode, so it would be just adapting a viewer for the files.
It would be a very interesting feature, which probably would help us to make Rockbox well known. I hope someone decide to do this little work (I just don't know how).