Hey, look at the bright side. They left the series off on a note that if the series did sell well enough, they always could make a Double Arts 2. This could very easily happen. Especially if they need a new monthly manga for Jump Sq. Just a thought.
i would absolutely love to see a DA "Revisited" sometime down the road and continue the story. although the "ending" is somewhat "revealed" i'll have to see the Edited Manga to see what i can make of the ending and if he left it open enough to possibly continue, or of course, he could consider this a "false ending" and pick back up right where he left off. which would be astounding IMO.
and with the way he ended the series, it looked like he was still ready to continue the story, but was told to end it when he was still working on it the chapter. or possibly he had already finished the chapter, but because of the cancellation he had to change the chapter after the fight to wrap up the story.
but here's some of the things he left open:
Hine's Painting (though we can guess what it's about...

)
Gazelle's identity & purpose
Andie Frau
What sister Martha and what the Sister's HQ looks like
how they find they cure/ antidote to Troy
among other things i cant even think of at the moment