I thought this was an interesting question for Funimation to ask, considering they are a localization/licensing/dubbing company. I didn't think they had much say in what Japanese companies choose to animate. Anyways I had fun answering this question and went to town on it, I really had 4 suggestions (the last one I put in the comments at the bottom). Here were my suggestions:
(book) Ender's Game by Orson Scott Card
I'm pretty sure lots of us have read this amazing sci-fi/psychological book, it's pretty famous and I'm surprised there hasn't been any live-action adaptations. Maybe the author is against it, but I think it would be perfect for animation.
(visual novel - DS) 9 Hours, 9 Persons, 9 Doors
This is one of those rare visual novels to make into the U.S. and damn was it good. The puzzle element was really cool too, it made you feel like you were smart when you finished them (especially the sudoku puzzle at the end of true ending)
(video-game - Xbox360) Eternal Sonata
A video-game about Frederic Francois Chopin (google him) on his deathbed where he dreams up an entire fantasy world all about music! The game's graphics are beautiful and of course, the music is stellar as well. Definitely good anime material.
(book) The Catcher in the Rye by J.D. Salinger
I'm sure a lot of you have had to read this guy back in High School. Most books we had to read in High School sucked, but this one was awesome. I felt it accurately portrays teenage angst, emotion, and arrogance. Also good anime material.
What did you guys suggest?