Back on the official Nsider forums, I made a thread for what I wanted Luigi's Mansion 2 to be.
Basically, it takes place at the same time as Super Mario Sunshine. While Mario, Peach, and Toadsworth are on vacation, King Boo sees an opportunity to usurp a throne and reestablish himself. Luigi, who is house-sitting, thinks he has a big beautiful castle to himself for the week. But when Boos suddenly swarm the castle and put the castle staff to sleep, Luigi hides in a castle closet and avoids the commotion. In the closet, he finds the in-home model of the Poltergust. He reluctantly suits up.
So instead of a haunted mansion, it's the haunted Princess Peach's castle, taken over and redecorated by King Boo and his minions. Throughout the building, Luigi can also enter the nightmares of the castle staff. If he eradicates the Boo that is giving them bad dreams, they staff member returns to form and offers Luigi clues and assistance. Luigi cleanses the castle of its spooks and haunts just in time for Peach and crew to return. Not knowing what he's been through, they thank him for house-sitting with a kitschy little souvenir.
In my heart, this actually happened.