In the volume 4 author's note, he said "I'll have to part with these characters until the next time I continue this story," so there's hope.
Anyone else want to see his one shot "Bad Shield United" as a full manga? Kubo said he wants to do a series based on it.
Also, a bit of trivia: Miyuki Eto, the author of the Jigoku Shoujo manga, said that the manga she is most impressed with is Kokumashi Urara (Rune Master Urara), which appears in volume 3 of Zombie Powder. He also wants to do a series based on that one shot as well.