Super Mario 64 2, intended for the doomed N64 Disk Drive. It was to star co-op multiplayer, and a playable Luigi, though all what was revealed is a pyramid level screenshot. Even before that, rumors started going around that the original Mario 64 was named at one point Super Mario FX (actually a code name for the FX chip) and was destined to the SNES before being completely scrapped. This was proven to be false though.
Urban Legend of Zelda: The "L is real 2041"/"L is real 2401" plaque on the statue in the castle courtyard's pond (where you can also find the Boos). Despite being very fuzzy, this was commonly interpreted as some way to unlock Luigi (who ended up being playable in the remake).
Then when the DS version came out, everyone thought that since Wario and Luigi were in it, Waluigi had to be a hidden unlockable character.
This message is also thought to say "Eternal Star." Incidentally, Eternal Star would be the final board in Mario Party.
Someone once said that the 2401 refers to the date Paper Mario, which Luigi was in, was released in North America. According to The Other Wiki, however, it was actually released a day later, on February 5th, 2001, but it's close enough, right?
What Could Have Been: Early previews imply Bowser was going to have the ability to shoot ice from his nose. This would later be given to the robotic Bowser in Luigi's Mansion.