Trinity Blood. I know the creator passed away, but that Anime could use a reboot plus a sequel. They finished with Abel and Ion setting out to find Cain. It really felt like there was more to that story. I do enjoy this series in all its forms, and some of the characters could really use some more fleshing out on the Anime side (Hugue and Leon come to mind).
Come to think of it, a prequel would also work, though I think it would have to garner appeal as a Manga first, seeing as how ALOT of prequels have proven to really suck.