With this latest chapter there is a greater chance the redhead character we see in the background is the dragon devil fruit user.
Everybody in the colorspread has a connection to Ryuma.
Everyone in front of the flame are descendants of Ryuma. Except for the fox which is one of his descendant's pets so there's still a connection.
The dragon we see in the background is more than likely the dragon Ryuma defeated. All that leaves is the redhead character. It wouldn't make any sense for Oda to put a random character in the colorspread that has no connection to Ryuma. That means he has to have a connection somewhere. A logical explanation at this current time is that character is the dragon and the dragon is a DF power. The dragon is a western style dragon and look similar to Loki's dragon df power.
I'm not saying this is 100% accurate. I could be wrong. I'm just basing it off the information we currently have.