The universe of DTB has a lot of potential, because of the choice Hei made at the end of the series to basically not fix anything, just exacerbate the situation by allowing contractors to remain in the world, after the incident which has made them public.
However because of all that, and the choice to remain as he is, one wonders if Hei's story is, ya know, already told.
I mean "suddenly revenge from the past!" isn't enough to carry a whole plot on it's own, it's more like a plot arc during an existing story. I mean it didn't work for Nadesico: Prince of Darkness, after all.