It was never planned to be a Gamecube add-on. True, Nintendo experimented with motion controls on the Gamecube, but that was before its release, just like they experimented with stereoscopic 3D on the Gamecube before its launch, but the 3DS wasn't originally a Wii add-on. At that time the motion sensors that were consumer-priced were too expensive and had too much lag to be usable, so the motion-controls were put on hold.
The Wii's low power was a result of Nintendo's decision to release a game system far cheaper than the competition, and the thinking that developers already knew how to program for the Gamecube so compared to the competition the development costs would be way low.
Rumors say the Wii-U has enough power to produce the highest quality PS3 and XBox 360 graphics, but at 1080p at 60fps. Which is plenty of power, because it costs HUGE development costs to make art even as detailed as the "lowly" XBox 360/PS3 can do. I mean think about it, why doesn't every game on PS3/360 look like Uncharted or Gears of War? Because those games cost a lot of money to develop. So think, if the next-gen consoles are capable of way higher quality graphics, most games will still look like they do now, because most developers aren't willing to put over $10 million into every game they make.
So as long as the Wii-U can handle the best the PS3/360 can do but at 1080p/60fps, it will be graphically competitive for all but the biggest games.
And we'll have Donkey Kong with Uncharted graphics, Metroid looking like Gears of War.