Super Mario 128 was a canceled game sequel for the Nintendo GameCube. It was planned to be the GameCube sequel for Super Mario 64, but was canceled for unknown reasons. Various elements of the game were implemented in various games over the years like Pikmin and Super Mario Galaxy.
The first mention of the name Super Mario 128 was in 1997 by Shigeru Miyamoto as a possible sequel to Super Mario 64. Also known as Super Mario 64-2 at the time, it was rumored to be an expansion or sequel to be in development for the Nintendo 64DD. Miyamoto mentioned in various interviews that the game was in early development but, the main focus was the 64DD's ability to store information and a cooperative mode featuring Luigi. Even by 1999, the game had not progressed much with the game only consisting of a single demo level with the cooperative multiplayer being the thing Miyamoto wanted to implement the most.
The game was first shown off as a tech demo for the Nintendo GameCube in SpaceWorld in August 2000. The demo started with large 2D Mario being split off into smaller Marios on a larger rounded disc. The Mario would move the boxes until it rendered 128 Marios walking about.
The game then submerged for sometime with Miyamoto later confirming that Super Mario Sunshine and Super Mario 128 were separate games at SpaceWorld 2001. The game was only mentioned in interviews that it was still in development from this point forward. In 2005, Miyamoto said, that the game had moved from the GameCube to the Revolution.
However, this story changed in 2006 in the September Nintendo Dream issue with an interview with Miyamoto. He stated that he had forgotten whether it had been prototyped for the 64DD and that the demo's technical efforts had been implemented in the other games.