I think that there are many different ways to look at what the next generation Wii system will look like ...
Hardware: I'm pretty sure that Nintendo is very happy with the processor core and GPU they have developed with the Gamecube and Wii, and it will likely be the base of all of their hardware going forward; their next generation handheld will potentially have very similar hardware to the Wii and will likely take advantage of a 45nm process.
For the CPU I anticipate Nintendo will move towards a asymetric multi-core architecture, where the base of each of their cores is based on the Gekko/Broadway core but that each has an extended instruction set to handle specific (game related) tasks. It shouldn't be that difficult to produce a core that is specifically designed to handle advanced physics calculations, animation, AI, or however they think the breakdown of work should be done. The processor's die size should be reasonably small (between the Gekko and Broadway) and the clock speed will probably be unimpressive (2.5GHz to 3GHz) but its design should enable it to perform at an excellent level for its cost and power usage.
I think the GPU is harder to predict and I'm not (really) sure of the approach they will take.
Overall I anticipate the performance of their system will fall between the XBox 360/PS3 and the XBox 720/PS4, but for its initial price of $200 to $300 it will be a moster (and most people won't notice the difference in power).
User Interface: I anticpate that Nintendo will move towards perfecting the Wiimote interface and will probably drop the remote-shape and will potentially gain buttons in an asymetric layout (like the Gamecube controller), the nunchuck will probably have a similar shape to the Wiimote and will lose one button in exchange for it's own "sensor". The sensor bar will disapear and the pointing fuctionality will become pixel perfect. Nintendo will include a head-set that is similar to the XBox Live headset and many games will take advantage of voice communication (people will talk about how revolutionary these games are).
