I have an idea that I would like to see implemented, forgive me if this has already been mentioned.
You know how the GBA contains the GBC sound hardware? Which allows it to not only play GBC games with sound, but many GBA games uses the GBC sound hardware (in conjunction with the GBA sound, of course). Well I was wondering, since the DS phat and DS Lite contains GBA hardware, if it would be possible to use the GBA's GBC sound hardware stuff. I noticed when playing the official GBA port of Mother 1 (the "Mother 1 + 2" compilation), and comparing it to the NES version of Mother 1 (aka EarthBound Zero) on nesDS, the sound on the GBA version sounded a lot crisper, and I preferred it over the sound on nesDS.
Obviously this would not be an option for DSi users, as the DSi does not contain a GBA.
(Note: The GBA port of Mother 1 does not contain any graphical or sound enhancements, it is a straight-up port of a NES game)