I believe there is a column in the SCBWI newsletter each month on this topic, too.
In my library, I could use:
More collections of scary stories (a la "Scary Stories to Tell in the Dark")
New Year's Eve/Day (Jan. 1, not Chinese New Year)
Phonics/Learning to Read (not Beginning Readers, not textbooks, something in-between)
The human brain, reflecting new research findings
Food chains and life cycles
Biomes besides the rain forest
More iterations of new baby books (parents usually want it to reflect their specific situation, aka big sister getting ready for new baby sister)
More iterations of death in the family books (same reason as baby books--aka boy experiencing death of grandma, etc.)
More research books for science projects (aka we'll have lots of books on chemistry science experiments for grades 4-6, but few books with plain chemistry information these experimenters could use as resources)
Building stuff with Legos--model diagrams
More ancient civilizations besides Egypt, Rome and Greece (such as China, Japan, the Aztecs)
Just a few!