http://www.sharp3d.com/technology/howsharp3dworks/This is a link to a kind of 3D display technology. The major con of this technology is that you need to be looking at the screen in a certian way. aka dead on. In the portable arena you are almost always looking at the display dead on.
Another thing is that this is a technology that because of the nature would not lend itself well to a 2 screen unit. Hence it couldn't do well as part of the DS2. Also this technology is not practical to use in TV's and for general computing.
You can turn off the feature to enable it to be a purely 2D diplay, playing older games. The prices of the computer display of these units have dropped recently and I don't beleive that the GBNext is going to be announced until nextE3 at the earliest or the one after that. Other things that I think are possible in GBNext is:
Analog Stick. Think the PSP numby only good.
A total of 8 buttons. 2 shoulder, 4 action, Start select.
Wireless. With Link cable emulation for older games.
1 gig carts. With the ability to use the unused space like system ram. (For decompresion)
Virtual Console interoperbility.
I think these feature sets would differentiate it enough from the DS to justify there co-existance.
Plus come on true 3D display without looking stupid.