In Super Mario Bros. U for the Wii U, there is (of course) a hidden World 9 called Superstar Road that you can unlock for some more crazy, and difficult levels to beat.
Unlocking the secret World 9 Superstar Road is the same as unlocking the hidden World 9 on New Mario on the regular Wii as well.
To unlock Superstar Road, you’ll need to get every star in all levels of all Worlds from 1 through 8. This includes any hidden levels in a world as well.
Once you collect all of these stars, and beat the game, you’ll see the new path you’ve unlocked to Superstar Road.
Guide: Getting Started With Xenoblade Chronicles X on Wii U
A Link to the Past Walkthrough – Ice Palace – 
A Link Between Worlds Walkthrough – Thieves’ Hideout – 
A Link Between Worlds Walkthrough – Swamp Palace – 
GOLY