I know my opinion is probably controversial, but I think Wii U will be the end of the line for Nintendo's traditional consoles. With the home console market the way it is, I just cannot foresee a future in which Nintendo makes a huge box with the capabilities of a PS4 or Xbone. For one, they are still a relatively small company, and their ability to constantly pump out AAA games for such a device is questionable. They also don't seem to have a real interest in chasing such horsepower (as seen with 3DS and Wii U). Few of their games would actually end up utilizing it anyway (Zelda, Metroid, maybe 3D Mario), so what is the point? Addionally, the home console market is moving towards convergence devices - basically PCs for the living room and even some cable box functionality thrown in there in Xbone's case. Nintendo have a long journey into uncharted territory if they are to follow suit.
What I do see happening is a complete paradigm shift in their device strategy. Basically, I can see them imitating Apple in sticking to one "platform"/OS and having a variety of devices which function on that platform. So, they could still release a dedicated portable and home console, and add even more devices in there like a tablet, but they would mostly play the same games. The architectures of such devices would be largely the same - most likely ARM-based CPU and some type of mobile GPU like Tegra or SGX. What would vary between devices is the number of cores, RAM, clocks, screen size, internal storage, etc. I picture their next home console as a small Apple TV (or Wii)-like device, eschewing horsepower for ease of use, unintimidating form factor, and low price. A variety of controllers would connect to it as well as their tablet and portable devices for dual screen gameplay.
The benefits of this approach are:
a) A streamlined output of their main franchises, preventing series fatigue.
b) Freed up resources to create more experimental titles
c) A wider combined userbase for most titles
d) Less risk. If one device bombs, little is lost as the games and architecture would be common to all devices.
e) By refreshing form factor periodically (as they already did with the DS and GBA line), they can keep up with consumer electronic design trends without splitting the userbase
Wii U BC could be supported in one SKU of the home console device by including the chipset, if it's deemed worth the effort. It would need to be distributed in appropriate quantities and sold either at profit or at cost, so that little risk is associated with it, and then likely phased out over time completely.