I think there are two major directions Nintendo can take:
1. Optical media
2. Matrix ROM media
I think backwards compatibility will be important and is possible in a portable based on either format. Here's my proposal for the optical media based successor to the GBA.
1. Make the mini-optical drive modular like on some laptops. This way you just slide out the drive and slide in a GBA cartridge adaptor module for backwards compatibilty with GBA games.
2. The optical drive could be the same drive used in the GCN except more compact so it could use the same GOD format.
3. The chipset would be a low power derivative of the GCN's or an ARM/SH-5/X-Scale/PowerVR MBX/Acceleon combo.
1. Optical media
2. Matrix ROM media
I think backwards compatibility will be important and is possible in a portable based on either format. Here's my proposal for the optical media based successor to the GBA.
1. Make the mini-optical drive modular like on some laptops. This way you just slide out the drive and slide in a GBA cartridge adaptor module for backwards compatibilty with GBA games.
2. The optical drive could be the same drive used in the GCN except more compact so it could use the same GOD format.
3. The chipset would be a low power derivative of the GCN's or an ARM/SH-5/X-Scale/PowerVR MBX/Acceleon combo.