Super Mario 64 2

Like many popular games, Super Mario 64 was planned to have a sequel, but it did'nt get released for a couple reasons.

Development and Demo Level
Super Mario 64 2 was meant to be released for the 64DD, the Nintendo 64s disk based add on. However, due to both the 64DD failing and the project still being in a reletively early stage of development, the game was canceled.

There was a demo level presented in which 2 players would play through a mission in a stage. Not much more is known about the game than it supported 2 players.