The sequel might be on its way...
Hey... wait! Keep those pitchforks away from me!!! I'm telling the truth!
Ok, LucasArts has nothing to do with this, allow me to explain:
Have you ever heard of the Adventure Game Studio? It's a free engine used to create adventure games (duh!) with no need of knowing anything about programming. Fan-made sequels to Sam & Max or Zak McKracken have been made with this engine, and they were damn good since you can actually rip the sprites from another games (plus they had good stories).
Now, I checked the forums recently and there is some user interested in creating "Grim Fandango 2". It might take some time since you can only create 2d games with that engine (there are other engines to create 3d games, but they are far more complicated).
For further information, go to bigbluecup.com, then go to the forums and search for "Grim Fandango".
Keep your hopes (and your shotguns) down. This is only a possibility.