More than whatever technical specifications the actual console has, I would be much more interested in figuring out what sort of marketing strategy they would employ. They have done a brilliant job of anticipating what potential consumers might want and providing that. If they just go: ''If you enjoyed wii, get ready for wiii2!'', I don't know if that will have such an impact as to allow them to DOMINATE like they have this generation. I agree with De85 when he states that they will have a much harder time doing that next time around, simply because I highly doubt their competitors will be caught with their pants down again.
A lot of people that appear to know more about this industry than me claim that the games are what matter. As we've already seen, if machine specs were all that mattered, PS3 would have destroyed everyone else this generation, but that has not been the case (at least yet).
You can bet your house that we will see new iterations of well-established franchises again, such as all the Mario, Zelda, and the rest of the usual Nintendo menu. Will we see the succesors to wii sports and wii fit? That's a question I would need a bit more time to think about. Sure, they can add prettier graphics and more events to play for those titles, but if they do their revolution thing again and come up with a completely different control scheme, they may have to rethink some of those ideas. Analog control and rumbling features were succesful and have been implemented by others, so I'm pretty sure motion controls are here to stay as well, at least for Nintendo Platforms from here on.
So it's the games that intrigue me. What will they come up with next? Ok, I'll admit the machine itself and the control system it employs has a direct influence on the games, as things like wii sports or wario ware would not have been possible with a traditional approach.