Enterprising fans have designed and released a full 80-level sequel to New Super Mario Bros DS, known as Newer Super Mario Bros DS.
The game offers 80 new stages spread over eight maps, and features a mix of original assets, and assets from Nintendo's 2006 New Super Mario Bros DS game. There are new and updated game mechanics, a returning power up, and a new (and faintly shrill) soundtrack featuring both original and Nintendo compositions. If nothing else, it's an impressive undertaking, and looks incredibly, authentically slick in the accompanying launch trailer.
Newer Super Mario Bros DS was made using the New Super Mario Bros Editor (AKA NSMBe), a popular community tool for PC that released long before Super Mario Maker, Nintendo's own, official level editor effort on Wii U. As such, a ROM of New Super Mario Bros DS and a compatible emulator for Windows, Mac, or Linux is required to play the game, immediately placing its use in legally difficult territory.
Nintendo, of course, rarely takes kindly to unofficial fan games making use of its copyright protected IP. Just last year the highly-anticipated, decade-in-development Metroid 2 fan remake AM2R was hit with a cease-and-desist order soon after its release.
It will be interesting to see if Newer Super Mario Bros DS suffers the same fate; the team behind the game previously created Newer Super Mario Bros Wii, an absolutely massive unofficial 128-level sequel to the 2009 Wii game playable on a homebrew-enabled Wii.