Nintendo hasn't made a numbered Mario game (unless you count Super Mario Galaxy 2) since 1988's Super Mario Bros. 3, but they revealed in January that they're working on a new, old school Mario platformer. The game will be 2D and come out for the 3DS, meaning it'll likely have 3D visuals.
We don't know much more about it besides that it's coming in 2013, but a domain name registered recently by Nintendo may give us a hint: supermario4.com. Heading there redirects to Nintendo's main official site, so they're not trying to hide that it's theirs.
It could mean anything, though, including that they're just snatching up the domain to cover any hypothetical eventualities. Hopefully, though, they're making another game in the style of SMB 3—maybe even a sequel.
Miyamoto recently revealed that he's considering doing just that for The Legend of Zelda: Link to the Past, so it's not exactly implausible. What do you want out of the next Mario game? Let us know in the comments or on Twitter.