Same base features as now...plus:
*More powerful, better screens, maybe a little wider, yet still as light.
*512mb built in flash memory.
*SD Slot, built into the GBA slot. Many machines now have one slot that actually takes multiple types of cards. I see the same thing here.
*I was also thinking maybe motion like the nunchuk, but on a handheld that probably wouldn't be a useful input...maybe.