Grim Fandango II
Well you never know. After all, Sam and Max is getting the 3D treatment after all these years, so why shouldn't Grim Fandango get a sequel?
I have to disagree though Timfandango, there's a lot of room for a sequel in the plot- it's not a matter of what happens before the Ninth Underworld, it's a matter of what happens afterwards. Something was rotten in the Land of the Dead, why not in the Ninth Underworld?
As for the better graphics etc thing, I'd love it (especially if you see the concept art for the original game, if it looked like that it'd be great) but they'd have to be very, very careful not to end up losing the game's very unique charm with faceless modern graphics.
I like your extended GF idea though, with the exception of the Christmas Party- some things just wouldn't be funny if they weren't a mystery. But the whole 'in between years' thing would be great, definitely.
Finally, Theodin, I like the name (fits a potential plotline excellently) but I don't think an entire game could be based around the fairly vague concepts you've suggested.