I can't really make a list of individual species, that would take too long, but a quick look through a field guide will identify birds in need ot this. Some general categories woudl include:
Loons and Grebes- Definitely needed, currently not included in illustrations
Herons and Reddish Egret
Ducks (most seem to be covered, although I'd be sure to get an illustration of them sitting on the water, as most people will ID them sitting on the water and not flying (which is tough)- for example CInnamon Teal male drawing is only flight and it doesn't give the oveall impression of mostly cinnamon that one gets seeing a bird on the water)
Hawks- many color phases and juveniles to be covered. Red=tailed Hawk, for example, is sorely lacking in many of the color phases, and it is a very common bird, although at least in many phases the red tail stands out.
Shorebirds- Sandpipers are an obvious one to show juveniles, especially because many have juvs that migrate later than adults and are by themselves when observed.
I'd say gulls, but that may be a bit beyond a program like this, it is such an advanced deal to peg all of the ages of these birds. Maybe later on down the road.
Alcids seem to be covered, at least the ones I glanced at. Very important to have breeding and alternate for these. Many people may go on general boat rides in places like Alaska or Maine and see one or the other.
Woodpeckers- Some juveniles would be nice.
Warblers- This is a biggie. Confusing fall warblers are high on the list, as are females adn juveniles.
Sparrows and Finches obviously, but I glanced at a few obvious ones and they seemed covered, White-crowned Sparrow for example had the juveniles included in the illustration.