Nintendo had a handheld console ready a couple of years ago but decided not to release it. This was probably a next generation Gameboy. At the time, Nintendo probably were still not entirely convinced the DS could continue their handheld dominance and were probably worried about the PSP, but Nintendogs, Brain Training, New Super Mario Bros. and Mario Kart DS put those fears to rest and another console was no longer needed.
With that in mind, the thought of Nintendo having a DS 2 and Wii 2 sitting there isn't hard to fathom but they are not the final boxes. Nintendo are continually making changes and adjustments as can be seen with the evolution of the Wii remote from TGS 05 to E3 06.
So in my opinion, we can't know what the next DS or Wii will incorporate as Nintendo doesn't even know. If it comes out next year then it may be similar in design to the DSi but if Nintendo waits even longer we could get something like this:
