Seconding Lichtenstein, especially in the style of the bigger, more surreal paintings he was doing in the late 70s (see Figures with Sunset: image-duplicator.com/main.p... )
It would be neat to have a series of levels, accessible by hopping into a painting or other secret entrance inside a museum, that took you through a progression of a bunch of the above suggestions, from the cave paintings onwards... I like the idea of going through a Dali melting-clocks-crutches-and-drawers screen that suddenly has flying dudes in the sky as the scene transitions to Chagall, or whatever.
I'm thinking Pete Fowler, for a keen sense of the kind of light-hearted cartoon grotesque that I'm thinking will be lurking in various places through Glitch:
Definitely want to second (or third?) the Miyazaki suggestion, and the Edward Gorey suggestion.
And obviously I just mean something in that style.
It might be hard to actually get Miyazaki himself to design it.
Ditto for Gorey. You know, the whole dead thing.
On a more realistic note, Der-shing Helmer, the artist of online comic The Meek, has a really individual, painterly style and I'd love to see what kind of level they could produce. www.meekcomic.com/
Or John Allinson of Scary-Go-Round & Bad Machinery - he's used lots of styles in the past but currently he's in a really simple scratchy sort of palette that I think could make a very interesting level! www.scarygoround.com/