Ok, here it is:
-It snaps on to the bottom of the DS Lite (pegs into the screw wells, stuff like that.)
-Contains its own rechargeable battery with charging plug and hooks up to the DS charge port.
-4 speakers (2 left & 2 right) and 3 subwoofers for rumble and better sound.
-Amplifier IC's powered of the unit's battery.
-Some for of Slot-2 Device built-in, with exended slot to a convienient location.
-iPod dock, with switch for iPod-DS mode.
-Nice blue and tri-color LED's, tri-color for audio and blue for power.
-Meh, anything else I think of.
Building begins in the new year when I have cash.

Update:
-External mic. so you don't have to put your face in your DS

Update 2:
-A/V out fo iPod (not DS)
-Expansion port for fancy soundboard with knobs and stuff.
-Knobs convieniently placed on the sides abot the hands so you can change the volume with you thumbs.
-Of course, some form of DSerial Support with programs.
-A camera, using a special camera interface on this page.
-Fun temp sensor for the heck of it here.
-Polarity switch (hoo boy, may be more than one: 17-pin card slot) for different devices
-GBA Selector (dug out of my basement.)

Update 3:
-Built-in flash drive to hold all your PC programs.
-USB Charger

Update 4:
-Built-in NoPass

Feel free to submit ideas.
(Note: some features may be moved to the sound board)

Needs to be bought:
-Everything listed
-Two PassMe Boards (for camera+temp sensor)