Banana Fish definitely deserves its own anime.

Five Star Stories only had 1 movie but has had 12 volumes released. The problem is that Nagano is slower than paint drying with coming out with them so it would be an OAV series.

My favorite manga Gunnm/Battle Angel only had a 2 part OAV release covering up to vol. 2 of the manga but an anime adaptation would create a 3rd ending for the original series unless it followed the manga closely. It would have to be a reboot since the OAV introduced non-canon elements. The previous endings were:

-original manga ending (retconned by sequel manga Last Order)
-PS1 game Martian Memory version

And Kishiro now has a 3rd manga series, Mars Chronicle, which is supposed to wrap up the Gunnm saga after all these years

I've mentioned them here before, but Mohiro Kitoh's works that were adapted into Anime--Narutaru and Bokurano--deserved much better adaptations than they actually got. Some of his other works also look like they could have done with anime adaptations.

Psyren? I really enjoyed that Manga.