WMG: New Super Mario Bros. U

One level will take place in Princess Peach's Castle.
We know that part of the game's story will have to do with Mario and. co making their way back to the Mushroom Kingdom after getting thrown out by Bowser. By the time you clear the seventh world, you'll be closer to familiar territory (and from there you'll probably go through a few levels evocative of the previous home console installment, with the same music, scenery, and all.

Peach's Castle, now overrun with Bowser's minions, will serve as a transition to the final world (like the Mysterious Sunken Ship in Super Mario World). After Mario and co. storm their way through the castle, they'll find Bowser on his airship with Peach, about to take off to his lair. The heroes will grab on to it before it leaves, and after a quick auto-scrolling airship level, you'll begin World 8.
  • Kind of confirmed. You have an airship level after World 7's Castle, followed by 6 levels in world 8, the last TWO actually inside the castle.
There will be more hidden in this game than has been revealed so far.
Super Mario World had a lot of secrets in it, both revealed in the manual but hidden in-game, like Star Road and the colored yoshis, and super-secret, like the Top Secret Area and the Special World. I think in this game, Nintendo will return to form and have secret powerups and stages that aren't advertise as selling points for once. Then, when people discover them and spread the info virally, it will build more hype than any advertising campaign could. Some secrets that could be anticipated:
  • More suits that haven't been revealed yet, like bringing back the Penguin Suit, or having the Cape make a grand comeback in a secret world.
    • The Penguin Suit is confirmed to return, but can only be found in the Superstar Road, the bonus world of this game. (Its inclusion was kinda spoiled by Challenge Mode though.)
      • Propeller Suit, too.
  • Colored adult Yoshis with special powers. It's been revealed in a recent screenshot that all players get a Green Yoshi to ride this time instead of a different color for each player. Why would they do this unless they were hiding differently colored ones in later stages?
  • A truely hidden Secret World that requires actual searching to find, instead of just giving it to you for enough star coins and beating the final boss.
  • A special mode unlocked by beating the Special World. Just like in Super Mario World.
    • These two were kinda confirmed. Beating all eight secret levels unlocked... one more level. As a Call Back to World is that they're in the middle of the Star Road looking secret area. But unlocking it also gives you one of the Save File Stars!

Nabbit is a Rabbid
  • Come on, it makes sense. One of World 2's Toads even calls him a weirdo.

The Secret Toad House
......belongs to Monita. She's been following Nintendo characters for years, and analyzing them on their adventures allowed her to create Nintendo Land. The Secret Toad House is her current home base, explaining the billboard full of data on what you've done.
  • Or Nintendo Land could have been Magolor's creation and the Records Toad House is his research center.
Nabbit is a Toad.
Based on this piece of fanart, it's entirely possible he is not a rabbit at all, and his real face is hidden behind his bandanna.
  • And the Record Toad House on Secret Island is his. Heck, it's filled to the brim with stuff he stole! The question is, why does he keep all those records about you? Know thy enemy, perhaps?