Following the release of Super Mario 64 on Nintendo 64 in 1996, there were rumblings in the gaming magazines of the time that a sequel would soon come along on the same platform, or perhaps the (ultimately Japan-only) add-on 64DD platform. That game never materialized. However, as a new DidYouKnowGaming? video highlights in detail, Mario creator Shigeru Miyamoto was indeed hard at work on a project known as Super Mario 128 for many years. It explains that a variety of factors — but especially Miyamoto’s being stretched thin producing so many projects simultaneously — ultimately led to the game’s cancellation. But elements of the game seeped into other Nintendo games across GameCube and Wii.
It’s really worth watching the full DidYouKnowGaming? video to get the entire story, but at various times, Super Mario 128 apparently intended to include Luigi as a playable character swappable in-game with Mario. There were also plans for four-player gameplay at one point. As development dragged on without ever being shown in public, Miyamoto claimed it was because Super Mario 128 had so many new concepts that he feared other developers would steal its ideas if they saw them.
A spin-off project from Super Mario 128 known as 100 Marios, which also made the rounds in gaming magazines of the era, later got aspects of its design reworked to become Pikmin. Yet in March 2007, it was finally clarified that 128 itself was no more. However, some elements of its stage design were reworked to become Super Mario Galaxy on Wii. The DNA of this lost Shigeru Miyamoto game continues to flow through the Nintendo catalogue.