I learned a lot from the posts on this page that have really changed my opinion.
Previously I thought he would have a DF that controlled the weather, because of the lightning bolt that saved luffy and the "green wind".
However, I completely missed how Dragon picked up Sabo at Goa and somehow traveled to his ship anchored at Shimotsuki Village while STILL BEING ON FIRE. This is probably the biggest hint Oda has given to Dragons DF power, along with weather control hints. I think this must mean he can fly, and at a good speed. Also explains why he could save Sabo without getting wet(which is why he was still on fire).
I have two thoughts on him flying: wind control could allow him to fly, or some sort of mythical Zoan that both fly's and has weather control elements.
I like the idea of the vermillian bird, and the dragon mythical zoan. If I had to choose I'd vote for the dragon mythical zoan. Sabo has dragon attacks, most likely learned from Dragon and Dragons ship has a Chinese dragon on the side. While this could be because his name is dragon, I think it's more likely due to being a Dragon Zoan. This isn't definitive though, obviously.
Still could be just a wind DF as well, if Wind can be used to make lightning as some here have suggested.
I just think a mythical zoan would really fit Dragon. Dragon with a dragon fruit though may be a bit too much, even for Oda. With that in mind i've heard of another possible mythical zoan..
Fujin, the japanese god of wind.
Things I like are the Monkey esque appearance. Uses a bag full of wind, which could be why Dragon always wears a cloak. He also is often depicted with a red tattoo on his face like in the picture I provided. He is not Rajin though, who is the god of lightning. So perhaps he would have to conjure lightning bolts in a round-about way as described by proponents of the people who things he has the wind logia.
Still interesting. I think all of these could possibly be correct.