Nintendo won't be releasing a basic upspec Wii so they can market it as "HD" as HD isn't a marketing bullet anymore but rather the industry standard. Just try to buy a CRT SD TV at any major retail outlet if you disagree.
The main reason for having a console that maintains rough parity with the PS3 and 360 is for the purpose of cross platform development. As long as Nintendo has a standard controller available that doesn't require a complete retooling of the user interface, then cross platform ports won't require a great deal of effort.
While that may not matter to the big time Nintendo fan that has convinced him/herself that all the major franchises that have been selling on the PS3 and 360 don't matter, to the average gamer they do and if you have to be a single platform gamer, most will want those games without having to hope for a late or downscaled port that in most cases won't even be developed.
As for whatever else Nintendo decides to throw into their next platform is anybody's guess if you're not privvy to that insider information. They won't abandon motion controls, but expect to see a fair amount of refinement in the motion capture system they implement that will likely combine technologies utilzed by Kinect and Move. The controller itself should bring back dual analog sticks for the purpose of cross platform compatibility.
Spec-wise, it will be dependent upon available parts at current market cost in order to offer a competitive price (under $300) while maintaining decent profit margins on a per unit basis. At minimum 1GB main system RAM, 512MB VRAM. At minimum. It goes without saying that they'll be using a multicore CPU with an AMD derived GPU based off their current designs being the most likely.
