The one I would really like to see in anime is Ga-Rei. I mean, there's a Ga Rei anime, but it's a prequel of the manga. I really want to see a Ga Rei anime, based on the manga ...
[EDIT] There's an other manga I'd LOOOOVE to see in anime, : JACKALS, by Shinya Murata at scenario and Kim Byung Jin at drawing, edited by Square Enix (and Ki-Oon in France).
It's ultra violent, the drawing is extremely dynamic, the story is.. well, totally adult and mature (not sexually speaking, though...).
If you like stories of vengeance, bounty hunters, in a Victorian London style city, then this one is for you!