Wuhu Island has always been about as confirmed as Skyloft was/is. There is no way that stage isn't in Smash WiiU.
You know, I'm not shocked by this stage, although the fact it is 16-bit is interesting to me.
In the past few months I have stated I felt that the Super Nintendo/Game Boy era is now truly "retro" (20 or more years old), and thus it could get the retro treatment like what the NES got in Melee and Brawl (where we had 2 8-bit stages in both games, and several more NES-era [inspired] stages in both game as well).
In other words, a Super Mario World stage (probably Bowser's Castle), a classic styled/inspired Mario Kart stage (Rainbow Road with inspiration from each Rainbow Road incarnation please!), a Super Mario Land stage, a Pokémon Red/Blue/Green/Yellow stage, a Super Mario Land 1 or 2 stage, an original F-Zero stage, and yes, a Star Fox (SNES) stage are all stages to keep an eye out for.
Personally speaking, I would love to see a 16-bit EarthBound stage on Smash WiiU, specifically Saturn Valley or Threed.
Edit: And with that said, I am off to bed. Adios Amigos!