On a whim: five manga, from my previous reviews and my own personal-faves shelf, that I think would make good candidates for animated productions.
Kurohime. The first few volumes of this were rough going for me, and I confess I didn't see the appeal of the whole thing at first. But then it settled down into a story that was less tail-chasing and more relentless forward evolution, and I got pretty hooked. An animated version could clean up the mess of the first few books, tighten the chronology a bit, and give us a pretty wild ride.
Ochō the Ear-Cleaner. Another as-yet-untranslated series I need to discuss when time permits. It's part romance, part history lesson and part frothy comedy, all set in during the latter Edo period and revolving around a woman with a skill-set hinted at in the title. It's nowhere nearly as silly as it sounds, and in fact quite a bit endearing. Think of something halfway between Oh! Edo Rocket and House of Five Leaves, if that doesn't sound like too perverse a description.
Kataribe. I was very impressed with this Masayuki (Moyasimon) Ishikawa title, a single volume not released in English but with the scope and fierce energy of a Miyazaki production like Nausicaa. A movie version would be a knockout; the manga's imagery gives only the barest hint of how this could look as a fully-animated production.
Yoshihiro Tatsumi's shorts. An anthology production of his shorter works, recently reprinted by Drawn & Quarterly, would be a neat project. The artwork doesn't have to be faithful to the original, if you ask me; this would be more of a showcase for his storytelling as filtered through the sensibilities of different directors.
Red-Colored Elegy. No deep review of this has been forthcoming from me if only because it's been difficult to describe without lapsing into a fanboy gush that serves neither you nor me. Imagine an Art Theatre Guild film (something of the same lineage as Throw Away Your Books And Go Out Into The Streets) rendered as manga, about the pain and ambivalence of being young, in love and futureless. [And so imagine my astonishment when I found it was, indeed, made into an OAV.] [Update: AniPages Daily talks about the OAV here.]