RUMOR: Nintendo developing sequel to Super Mario Bros 3

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Soundwave said:

Wishful thinking. There's no way a Westerner would have the goods on a game being developed under Nintendo directly in Japan.

NCL doesn't suffer leaks.

Besides Super Mario World IS Super Mario 4, it was even labelled as such in Japan, the reason they used the title "World" is because the number 4 is considered unlucky in Japan. 

I could see New Super Mario Bros. 3 happening next year though for the 3DS. Next year has a dearth of 3DS content, NSMB3 + Zelda: Majora's Mask (another rumored title) would certainly help fill things out.

Smash rooster and Mario Kart DLC get all leaked.