Wii U FEELS like a Gamecube successor to me. Which is a good thing; Gamecube had amazing games and was a superb system. The Wii U has amazing games and is a superb system.
You may say that the same games are on it as the Wii, that it's underpowered, has a 'weird' controller etc. but ultimately you buy a console for the games, and the games are more along the lines of the Gamecube than the Wii. Ergo Gamecube 2.
In terms of philosophy I think that Nintendo planned from day one that the role of the Wii U was to bridge a gap between them and Sony/Microsoft; the Wii U enables them to catch up with the advances in networking, HD development, e-commerce etc. that the competition made in the last generation. And I suspect that the successor to the Wii U will hit in 2017; at least 2 years ahead of the competition, and again be a generation and a half in order to acquire the latter half of the generation's multiplats and build a user base before PS5 and Xbox Square Root of -1 launch.
Assuming both Sony and Microsoft's game divisions survive the generation (my money is on at least one casualty, probably Xbox being sold off to Samsung or similar).







