ARES!!! MAKE A VAGABOND ANIME ADAPTATION, AND MY LIFE IS YOURS!!!
I was gonna suggest most manga that Inio Asano has done. I can't see them adapting something like A Girl on the Shore and especially not something like Oyasumi Punpun. Solanin was adapted into live action and not an anime. The only anime that was adapted from his manga was Dead Dead Demon's Dededede Destruction which I guess fit an anime the most out of his manga.Also, Oyasumi Punpun but I doubt that they'd get the tone right and wouldn't want to see it's legacy tarnished.
Both are great mangas, Sanctuary is massively underrated. One of my favourite mangas is Chronowar by Kasumasa Takayama, a very obscure autor who blends sci-fi with existentialism, is really good. Plus 2001 Nights by Yokinobu Hoshino have an OVA, but not a full adaptation of the manga (another favourite of mine).Gunsmith Cats and Sanctuary, both have OVAs but they are very short and cover little to none of the manga. Sanctuary is also written by Buronson who also write Fist of the North Star.
I will also throw GTO out there, it has a pretty good adaptation but it stops about half way through and makes up it's own ending....not to mention the prequel/sequels that were never(or barely in the prequel's case) animated.
Code Geass: Soubou no Oz, and Code Geass: Shikkoku no Renya.
Some of the spin-offs/prequels for Great Teacher Onizuka.
Technically they have anime, but the second half of Akira was never adapted. Nor was the second half of Deadman Wonderland. Same with Ben-To beyond the first season. Same with Gunsmith Cats. Same with Bobobo-bo Bo-bo-bo.
Also a novel that got a manga adaptation, but no anime: All You Need Is Kill