In an interview with the Wall Street Journal, Nintendo head Satoru Iwata says that while there will indeed be games that hit the Wii in the future that require the upcoming Wii Motion Plus accessory, Nintendo hopes to make the add-on as inexpensive as possible.
There will be games that will be enhanced by the Wii Motion Plus as well as games that can only be played with it. Users will need four if they have four controllers, but we're going to try to offer it for as little as possible. We haven't announced the price yet, but the cost of making the Wii Motion Plus is not that much, so I think we can make it very affordable.
Iwata also says that, despite my theory on accessories and the Wii, Nintendo doesn't plan to release a whole lot more for their console.
Iwata also goes on to talk about their next console.
Mr. Iwata: The hardware team started work on the next thing as soon as they were done with their previous project, but what they think up doesn't necessarily become a product. We only turn something into a product after it's been thoroughly vetted inside the company. We're not at a point where we can give specifics, but of course we're working on it.
I see Nintendo riding the Wii wave for quite a long time, what with those developing countries to still tackle.
Keeping Up Nintendo's Momentum [Wall Street Journal]