I hear the 1st game was pretty short.
Whats the chance of them NPCing the first game, but then adding a whole Wii specific 2nd game(expansion) to it of the same length.
a 2 in 1 experience.
This is Nintendo we're talking about, a company that won't lift a finger to do anything with any of their games if it doesn't guarantee at least a 3-fold return on their investment (in other words, "they're pretty damn cheap"). My guess is that you'll buy it and be thankful you got it, or at least that'll be their attitude if it happens.
But yes, Luigi's Mansion on the GameCube could easily be completed in a couple hours. If Nintendo wanted to, they could probably easily code a 2nd Quest or something using existing assets, but I doubt they'd go to the trouble to do so.