The Suicide Theory[]
It is shown that whenever Luigi was talking to either Toad or Professor E. Gadd in the Telephone Room, lightning would strike the room, and it would reveal a silhouette of Luigi's body being suspended in mid-air. It is believed that Luigi had actually committed suicide by hanging himself from the ceiling, and that Luigi's Mansion is chronologically the last game in the Mario franchise.
Bogmire[]
The boss Bogmire resembles the Spoils Bag from The Legend of Zelda: The Wind Waker; this could
possibly mean that the Spoils Bag inspired Bogmire.