I personally love the Atari Video Music, PS1 music virtualizer or the JagCD VLM.
http://www.youtube.com/watch?v=BX1LVBLUYNs#
http://www.youtube.com/watch?v=XsWT33QzrLk#

So as I have room left on a top secret upcoming mod for Atari consoles I have the idear to implement an adaption of the Atari Video Music device. The solution is quite easy to implement and costs could be near zero giving a nice function to an Atari 2600.

The idear is to offer an additional connector on the AV/Audio cable to input any audio stereo source. If you switch the Atari 2600 on and no game is inserted the internal firmware menu starts offering the video music feature as well. By moving the joystick you are able to alter the visual output.

If activated any music lead to visual output like shown here:
http://www.youtube.com/watch?v=etZVnILnoSg#

So worth to implement? What do you thing about it?