Way-off feature?
This isn't that way off, but isn't it time they made a fun, easy VOIP walkie-talkie type of app for audio fun between two DS units?
Except not requiring internet but just used for local chats, intercom-like between rooms and stuff? Buzz other people and get them to join in a chat or simply leave notes on their DS? Of course wireless has to be kept on for this to work and it would ideally be enabled when in a low-power off-but-not-quite-off state, too.
Mum (who loves her DS) can also buzz Junior to come into the kitchen on their DSs. I know, I know... not exactly essential but a lot of FUN, right?
Something like this VOIP walkie-talkie app combining whiteboard-type drawing stuff with the stylus would be so much fun over wireless (and add to the 'social buzz' of the device).
Optionally it could be made to work over the internet, which would be killer, but they don't have to go that far if they don't want.
Cost? They would most likely have to license a codec or two, but the rest they should be able to handle, I think.