As I am both a planescape-fan and a pencil-wielder I'm really glad to see that you're looking for artwork. The idea of creating artwork for kind-of-official-planescape-goodness makes me feel warm inside!
But what kind of illustrations do you still need? Just name a creature, NPC or place or something and I'll draw.
Personally I'd like to see more art on Sigil. If that's okay with you of course.
I figured we might as well start someplace... I picked Ysgard b/c it's just complicated enough to be a challenge without being so difficult that it's asking too much. Helps me gauge skills easily that way. 
But other suggestions are welcome too. 
Im doing a picture of Ysgard at this moment, altrough I haven't included the world ash, is it as good as finished. Like terra nova said, some pictures of Sigil might be a better idea (there's a whole chapter of sigil to cover) or meaby some illustrations for by the feats and skills chapter?
Actually, if you are up for doing some landscape pictures, We could really use some of those. Go ahead and take a shot at say Ysgard? With a good view of the Tree there?
Also - let me know how much direction you want as an artist for working on this project. I'm more than willing to let an artist run free and wild as much as they like for what they want to illustrate or give direction if they want instead.