Actually, according to Nintendo's rountable, Next Level Games (the guys behind Mario Strikers/Mario Football and the new Punch-Out!!) will be developing Luigi's Mansion 2.
Anyway, I've always thought it would be cool to have a sequel to Luigi's Mansion. Shigeru Miyamoto said he would like to see a sequel about a year ago, and lou and behold, it was so.
I ESPECIALLY love how Luigi explores an Egyptian-like crypt. Multiple mansions means more haunted-theme possibilities!
I also like how Professor E. Gadd and King Boo are back, they're awesome and they haven't really done much of anything over the years.