Super Mario 64 2

Super Mario 64 2, or Super Mario 128, is a tentative title to cancelled sequel to Super Mario 64. It was set to feature Luigi as a playable character in the game's multiplayer mode, and had a mountable Yoshi. Originally planned for the Nintendo 64DD, it was cancelled when the console proved unsuccessful commercially. While scrapped, ideas from the game live on with future Mario instalments such as Yoshi being introduced as a rideable companion in Super Mario Sunshine and Super Mario Galaxy 2. The idea for Luigi within a multiplayer mode lived on through the New Super Mario Bros series.

Plot
Though no plot has been found, it would've most likely be like the first game with new levels and Luigi.

Why it was cancelled
The Nintendo 64DD was a commercial failure, leading this game only a bit of progress before getting flushed down.

Result
No game, but the closest that we'll ever get is Super Mario 64 DS, Super Mario Galaxy, and its sequel, as they have Luigi and the DS version has a multiplayer mode, and all have new levels.