At the very least, I wish they'd release special compatibility version, one that can specially work over a wide range of 3D cards and hardware, over all OS platforms. It's getting harder and harder to run this game that should be placed in an equivalent to something like the National Archives.
If they did make some sort of prequel (sequel is out of the question) I doubt they would still be able to include Manny or any one else considering how difficult it might be to round up the voice talent, plus doing prequels always some how ends up retconning something the wrong way.
However I fully support anything that at least takes place in the same verse, Mexican flavor and all. They very setting of GF could hold so many stories.
Also, I would oppose any attempt to make the game "with better graphics." GF is one of those few games whose graphics are literally timeless, thanks to the distinctive style Tim came up with for the world in resembling the paper mache dolls associated with the Day of the Dead. The only change I'd ask for is to make the invisible walls where Manny can't move beyond a little more distinct and far less buggy, but beyond that "updating" anything else would be sacrilegious.
No matter how good the graphics in games will get, you will always be able look at GF and say that it's beautiful