For another canceled sequel of Super Mario 64's, see Super Mario 128
Super Mario 64 2 was the tentative title for a Mario game in development that served as the direct sequel to Super Mario 64, intended for release in May 1, 1999. It was going to be for the Nintendo 64 Disk Drive, but was canceled early in production due to lack of progress and the commercial failure of the 64DD add-on.
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.
No game, but the closest we'll ever get is Super Mario 64 DS and Super Mario Galaxy, as they both have Luigi and the DS version has a multiplayer mode, and both have new levels.