Many rumors about the early, supposedly darker game are unsubstantiated, such as that the game was going to be T rated, and that the game was going to be a Timed Mission where Luigi had to save Mario before dawn or the mansion would vanish along with Mario. A timer for the Game Boy Horror is seen in some pre-release materials and Dummied Out in the game, but it was likely just for the playable demo at E3 2001 to stop people from hogging the game.
There is also the infamous Safari Ghost that wanted to kill Luigi and keep his head as a mount. He was supposedly cut for being "too scary". There's no proof such a character was ever in development. The legend is sourced from a caption in Nintendo Power, however the misleading quote came out after the game was already out in Japan.
The infamous beta "Game Over" screen in the demo version which shows Luigi as depressed that he didn't manage to save Mario and/or being possessed. This has been revealed by later findings to not having been a Game Over screen of the demo at all, but part of the E3 trailer of the game. It was never intended to be in the final product, and the circumstances of Luigi's state in said scene is left ambiguous.
The game was initially much harder. As seen in the trailer shown at E3, the Poltergust had an exhaust meter which, if overheated, could blow up and cost Luigi some life. The map that's included on the Game Boy Horror originally didn't have the feature to find the right room to fit the right key in.
Also, rather than being split up into different areas, the entire mansion would be just one huge area.
Luigi's Mansion was also supposedly planned to be in stereoscopic 3D using an add-on for the GameCube, but both the add-on and the 3D feature of the game were dropped when Nintendo realized that the cost of the add-on would have exceeded that of the base console itself (the add-on was supposed to go into the smaller second serial port, which never got used and was dropped in a console revision). The idea of Luigi's Mansion in 3D was finally realized when both a sequel and a remake of the original were made for the Nintendo 3DS.
Some Dummied Out ending files in the game are three graphics that show Luigi depressed while holding a flower, smiling with one peace sign, or smiling with both peace signs. This meant the game was going to have an End Game Results Screen, if not proper (or different) Multiple Endings.
A second version of Totaka's Song appeared as one of the songs Melody Pianissima could quiz you about. It and its related text is in fact still in the game and can be hacked back in.
There were going to be ghost enemies that would Jump Scare Luigi, briefly disabling him and crippling his health to 50. The concept was taken and nerfed for the Sneakers in Dark Moon.
A 2-Player mode was going to be programmed in the game but due to technical issues it was scrapped. The only remnants of this planned feature is a 2nd Mario model stretched out to Luigi's proportions. This was fully realized in the 3DS port, as 2-player co-op is featured, with the second player controlling a clone of Luigi named Gooigi.
In the game's dummied out files, there's a unused boss character named "elh", Its a tall, faceless, twisting creature. He has no textures or shadings but he has six animations and 4 particle effects within his files. It's unknown if he was meant to be a boss or an overpowered enemy seeing that he's the only enemy who is not a ghost plus his design does not match up with the games art style.