For another canceled sequel of Super Mario 64's see Super Mario 64 2
Super Mario 128 was a cancelled Mario game in 2000 for a testing to see how the GameCube Graphics can handle.
Plot (There is no actually a plot)
When the game boots up, it shows a pixelated Mario on the screen. Suddenly the pixelated Mario goes into a 3D state (which actually was 3D, but didn't knew it when it started) and bunch of Marios started appearing under the blocks of the pixel Mario, throwing them into the void. Once the counter reached to 128, no more Marios would appear. Then, there is nothing much that happened; The Marios were doing whatever they were doing, throwing each other at the void (Yep, Marios were harmed during this chaotic experience) and the Monopoly-like board was changing it's shapes, creating small mountains, and even BECAME A PIZZA! And that's it. Just showing how theGamecube would handle all of the Marios.
Why it was cancelled
It was a test to see how much the GameCube could handle on one screen.
They were working on other 3D Mario Games
There was never any leak on the internet of this game and everyone thought that this game after many years was cancelled, but Miyamoto, said in a conference that people actually played the game, into the form of Pikmin. So that means that the game was changed into Pikmin. (And later became Super Mario Galaxy too)