Well if you mean "story arcs" in a manga that I would like to see that was not in the anime then ALL the story arcs over the Budha alien mission in Gantz, The final arc or katekyou Hitman reborn.
But if you mean manga that has not had a anime at all than Ubel Blatt, Battle Royale, Aiki.
My Profile pic, Battle Royale. I believe, however, that will not happen. I just read where CW in America was in talks to turn Battle Royale into a Television show. However, we had school shootings here, and as a result, they lost interest in the project. Perhaps in the future. But I loved the films. For some, it is distasteful. But it is also a very real way to look at how we can become in a situation. I think that this forces us to think. It's a mistake to NOT make this into a television show or anime, because in doing so, we have a platform to address the issue without being 'preachy' and longwinded. If this would have been made a TV show, or anime, it could have been used to show how ugly we can become. Battle Royale is far from glorifying violence. In fact, it makes it distasteful. We watch, and are entertained, as it's fiction. Afterwards, we realize that it is not something that we want in the real world to ever happen.