I say it's never to early to think ahead, so that's what i'm doing. As i see it, Nintendo will probably do with the DS what they did with the Game boy and improve the handheld by adding new fetures, not making something compleatly new. I'm going to post my idea, and i hope that you will post yours. Oh, and please note that i don't know a lot about the technical side of this, like how powerfull the graphics engine could possibly be, ect.
The DSX would be a highly improved version of the DS Lite, and would have the samd basic structure. However There are fivechanges to the build though. Number one, there is an exra hole on the left of the DSX, which contains another stylis. Number two, on the back there would many tiny slots, because the game cartrige would be the height of a DS game, but much thiner. Because of their small size, you could fit a large number into your DSX at once, and then choose which one to play. And like the DS, the DSX would have a slot for past games to be inserted, and could play DS games, GBA, GBC, and the original Game Boy. Number three, there would now be 3 mics instead of one, offering a much more precise detection of sound. fourth, the top screen would be much larger now. Finaly, it would be a little bigger now. As you probably guessed, both screens are touch screen and can sense two points being touched at once, thus the second Stylis and larger top screen. Another very intresting feture is that the DSX has the ability to sense tilting in all directions. The graphics would be bumped up a notch to about Gamecube level, and we would finaly have the harddrive we've been waiting for. The downside is that all of these new fetures would make it cost quite a bit.