Luigi goes over to his new Mansion he bought with his money, and when he arrives he finds a letter on the ground: Dear Luigi,
I came over here to see if the Mansion was safe, and when I arrrived ghosts led by, someone familar, attacked me. I'm now being held captive. I called Mario and he tried sneaking in to rescue me, but he was captured as well. Thus, a green toad sent out paper and pen, allowing me to write this. Please defeat the ghosts and rescue us. I have no idea where we are. Just find the all-new poltergust 4000 in my lab and the Game Boy advance. Then you're good to go.
Sincerely, Proffesor E. Gadd.
Luigi goes around a 4-floor Mansion, with all-new ghosts and all-new puzzles. It's 7 areas long, and you can unlock mini-games in which you play fun, classic Mario levels as Luigi. If I have more ideas, I'll post them.
I kind of hope, after the events of Luigi's Mansion, Luigi and E. Gadd started a ghost hunting service(basically Ghost busters) and in LM2, Luigi starts off with small jobs and going into smaller houses and stuff. Then later on he's hired by a mysterious person to help him with his mansion.