I'm definately in favor of an "Unnatural Color" string of horses, containing a grulla, a dun, perlino, etc. I also like the idea of doing a set based off of very popular CS pets - that way pretty much everybody is guaranteed to like them.
Treasure Horses-Copper with Emerald eyes
-Silver with Sapphire eyes
-Gold with Ruby eyes
-Onyx with Diamond eyes
-Diamond with onyx eyes! lol
Weather Horses-Lightning
-Snowstorm
-Wildfire
-Rain
-Mist?
Galaxy Horses-Stars
-Comets
-Nebula
-Solar System horse (haven't quite thought of how that'd work)
-Moon
-Sun
-Meteor Field
-Galaxy (like a pinwheel galaxy on it's flank or something)
Famous Racehorses-Secretariat
-Ruffian (she's the whole reason I even suggest this lol)
-Secretariat
-Barbaro
etc
And Seasonal Horses; I love the idea of doing a set for the different times of day with the sun/moon moving; and the rainbow and mythological ideas as well. I was going to suggest Birthstone horses, until I realized there would need to be twelve seperate growing horses and that's just insane
Also, I haven't actually read the books but these new horses seem to be Arabians, so have you considered doing a set on The Black Stallion? Or, for that matter, The Saddle Club? I figure not, since these appeal to horse people rather than the whole CS crowd =)
I can come up with dozens of other ideas, but they're all either unpractial, or just plain stinky lol
-Chianya