Top of my list for picture books: a story about the Mississippi River. Of course! - Also eager for a story about a kid who collects (rocks, buttons or some such), and one about the beauty of the Western States landscape.
In an interview once, I mentioned that I adore opera. So, sometimes I get Mabel the Singing Cow kinds of picture book projects. Uh, uh. No. But I'd really like to see a picture book biography of Lawrence Brownlee (so many facets to explore here. One of the world's top tenors, and someone making great impact on African-American inroads in the classical voice world) or Renee Fleming (incredibly accomplished. The novel BEL CANTO was done with her advisement. She has sung in dozens of languages, including Elvish, for "Lord of the Rings" movies).
For middle grade, I'd like to see a contemporary novel in which the protagonist experiences a growing awareness of global or other societal issues as they dovetail with his or her own life.