Everyone already knows that there is a new gameboy in development.
One thing that I think is really going to determine this thing is price. Reading the specs on that thing, everything just sounds really expensive. If I have to pick between a $200+ PSP, and a $100-150 GB-X I am going with Nintendo.
Also I think it is important for Nintendo to be sure to keep up the backwards compatability with the gameboy. Use some of the really choice GBA titles to sell the system. It would help a lot at launch. Just as with the PS2 playing PS1 games. One of the biggest things that hurt the N64 was a huge lack of launch titles. I think there were like four games out for the first month of that system.
The GB/GBC were like portable a NES.
The GBA is like a portable SNES (but a little better)
If Nintendo can make a system that is as powerful as the N64, without some of that systems faults, I think they would be near unbeatable. With such a small screen you aren't going to be able to display much more complex graphics than that anyway.
In the past they have tried to keep the $100 launch price. I think they could moved it up to $150, but used that 50 to really flesh out the system and remove anything people could nitpick about. Keep is about the same size as the GBA SP (folding is an awsome idea, i say stick to it). Make sure it has a lit screen, headphone jack(no adapters), rechargable battery (the same or better life than the GBA-SP, or even an option to use one of those fancy micro fuel cells that are being developed for laptops! *drool*), a wireless link as well as wired one (gotta keep that for the old games/GC links).
I am sure Nintendo has already thought of all of these things, and probably some better ones. I just hope they make the right decision and go for the gusto instead of the bottom line. The gameboy has a history that cannot be undone, I just hope Nintendo doesn't think they are cemented at the top of the portable hill. If they aren't careful that great history might be all that is left of the Game Boy in a few years.