Luigi's Mansion/WMG

Roivas Mansion will be a bonus level in the 2nd game
It would be interesting.

The twins are the cause of the mansion's current state.
Odd and reclusive from the game's very description, the two kids were probably that way for their dealings. They may have found a book of black magic in the library and locked themselves in their room trying to figure out a spell. When they did, things went horribly right, resulting in some force overtaking the mansion and killing the inhabitants, thus completing the ritual and releasing King Boo. This is the LEAST nightmare inducing possibility under this premise, as it could also be the twins becoming possessed and killing everyone personally before committing suicide.

At the end of every mansion but for the last in the second game, Luigi will rescue a Toad
Each time, they will exclaim, "Thank you Luigi! But your brother is in another mansion!"

Luigi's mansion actually occurs at the end of the Mario timeline
I'm actually kinda surprised that nobody has guessed this yet King boo is actually the ghost of Bowser coming back from the dead to take revenge of Mario in the afterlife the game states that Bowser had died and King boo himself said that trapping Mario in a painting is revenge for all the troubles and plus it also explains why he has a giant robot of Bowser for the final boss additionally the 50 Boo's you capture in the game are the spirits of his long dead Koopa army

Luigi's been Dead All Along
The shadow "glitch" just seems too logical, and it would further explain this situation he's in.
 * Jossed, it seems like he's alive for the sequel.

Bowser has really been King Boo in all the later games.
Bowser had a Heel Face Turn and lets KB play him.

The mansion in the sequel will be Ashley's.
It'd be great to team up with her to get rid of the ghosts.
 * Maybe one of them, since the sequel will have multiple mansions.

Luigi singing the game's theme music is a form of Survival Mantra.
It makes sense, since he's visibly terrified throughout the course of the game. Why else would he be singing?

Chauncey died while still in the womb.
Chauncey's bio says he was born a ghost. However, this is impossible. This may be a clever way of saying he died before he could be born, perhaps intended to not freak kids out so much.
 * It's plausible that Lydia died while still pregnant with Chauncey.

Neville was a relative of Nigel Thornberry.
Notice the resemblance?