Using a tool called the New Super Mario Bros. Editor, a team of fans have released a fan game of sorts they call Newer Super Mario Bros. DS. This game comes in the form of a patch for the original game’s DS ROM file. This can then be played on a computer, for example.
This is a pretty expansive project, resembling more of a Nintendo-style pseudo-sequel than a novelty edit. It has 80 levels, eight worlds, and is even outfitted with visual changes and new music. This team is also responsible for a similar project released a few years ago, called Newer Super Mario Bros. Wii. The team has released a trailer for the project, and the download is available for free, of course. Figuring out how to get it to run is up to the individual, for obvious reasons.
In other ambitious Super Mario ROM-hacking news, a separate project called Super Mario Land 2 DX was also released as 2017 ended. This project is a hack of the Game Boy classic Super Mario Land 2: Six Golden Coins, which adds full color to the game.
Nintendo doesn’t seem to go after emulation hacks as aggressively as it does full fan games, and as of this writing both patches are still easily available.