Release date: app. December 17, 2015
To even highlight the characteristic of his DF, Jack even has huge tusks. Makes one wonder how heavy they are..
We also know Jack was trying to find a Ninja from Wano (most probably) and that Momo ate one of the man-made DF that was created by Caesar, who at that time worked for Doflamingo.
Many events are connected now, and looks like Wano is a country that plays a big role here.
It's also clear why Inuarashi was so thankful to Luffy now.
From this, the importance of both dukes is heavily emphasized and they will definitely play a big role that can even shake the whole world. It might be in the upcoming arc, or it can be later in the future. Personally, I have a feeling it will be later in the future though.
Here is also another interesting theory about them possibly being part of the Revolution army: http://www.onepiecegold.com/inuarashi-and-nekomamushi-are-2-of-the-revolutionary-leaders/2/