DS isn't technically a gameboy, they dropped the brand cos it had some sort of stigma apperanlty.
But there are other "innovative" features that can be added.
The DSi has a(2) camera(s), which can't currently be used for gaming, but do show that Nintendo could go for gesture based gaming in there next handheld.
The iPhone, which is vying to compete with he DS n PSP, has accelermoters like the Wiimote n PS3 Sixaxis, which has led to games like Super monkey ball where you move the iphone to move the character.
Also they could cop out n just stick two touchscreen's on.
So there's a wide range of place's to go, and ive only named the stuff that's out there already.