I think Wii Fit U and Wii Sports U will be a lot like Nintendogs + cats on 3DS.
Nintendogs (DS) -- 24.49 millon WW
Nintendogs + cats (3DS) -- 2.86 million WW
Brain Training on the 3DS at least in Japan also fizzled out compared to the older Brain Training games.
These things have their time (not even specific to Nintendo, you can look at Guitar Hero/Rockband too, Just Dance is probably on its way out in a year or two also).
I think Wii Sports U would do better than Wii Fit U though.
Also New Super Mario Bros. U *is* a "heavy hitter", lol, Zelda, Smash Bros., etc. are all smaller franchises. The only franchise Nintendo has that's reliably bigger than 2D Mario is Mario Kart.
I think Wii U is much more like the GameCube in appeal, it will center around hardcore Nintendo fans and kids as the main audience. But I think it will do about double the GameCube anyway with a better showing in Europe and Japan long term. Which is SNES territory (my guess is 40-46 million), which isn't bad.
