Totally enjoyed Grunty Industries - from the music, to the darkly toned and gritty atmosphere, to the maddening multi-floor puzzles. The way you enter Grunty Industries is simply awesome, as it made it feel like you had really accomplished something. I also loved the challenge of figuring out how to visit every room in the factory, and then working out how to obtain the various Jiggies locked away in the rooms. Maybe it's just me, but I'd love to see the developers pull another one of these.
If there is such a level it should probably take a few notes from Rusty Bucket Bay, because a factory by itself without something around the outside of it for you to explore would bring boredom to the environment, much like GI in Tooie did.
However, if there is a factory I'd want it to go all the way - a minimum of 5 floors, the rooftop, and at least one basement floor (a multi-floor basement could certainly add some creative depth!!).
A word on color: Perhaps use LED looking signs and the like, but not in a way it makes things feel "construction site"-y.
An alternate play on the Grunty Industries motif, for those who dislike the grit, would be a combination of that and a skyscraper/office building.
Frantic Factory wasn't all that much of a great world to me, but it did have good music.