Some obvious units that I don't *think* we have art for (my memory is fuzzy) and will probably need new models:
harpy
kislev war wagon, imperial war wagon (these are not the same IIRC, one is towed by horses, the other is self-propelled?)
inquisition pyre
Tilean Birdmen of Cartazza (leonardo da vinci type flyers)
genies, flying carpet knights
fell bats, vhargulf, black coach
winged nightmare
Gor's of various types
rat ogre, doom wheel, screaming bell
bull centaur, swivel gun, lammasu
foo statue, thunder drakes
oni (asian ogre), oni mage
ki-rin (mock up from a unicorn?)
wingless dragon/serpent, phoenix
engine of the gods, stegadon, teradon
various ogres, gnoblars
*edit*
updated removing some units
If you pick some you're interested in working on, we can give some art design guidelines.
Or take a look here:
http://www.games-workshop.com/gws/catalog/landing.jsp?catId=cat370003&rootCatGameStyle=wh
for tabletop art to base things on. (Go through the various Warhammer Armies tabs: Core/Special/Rare units.)