I would like to propose the theory that Monkey D. Dragon is a wind logia. The first point I want to address is when Dragon confronts Smoker in Loguetown. The weather condition present at Loguetown resembles the characteristics of a hurricane. If Dragon were able to control the winds then he would be able to dictate the direction of warm and cold fronts. When those two fronts merge together they may form a cumulonimbus cloud and produce tornadoes and lightning.
When Dragon saves Luffy by grabbing onto the hilt of Smoker's sword, the hilt seems to go right through his left palm (Not sure if it was intentional or if there was a mistake)
The tattoo on Dragon's face are in the shape of diamonds. According to this website on Native American symbols (didn't look to legit, but it further propels the argument. http://www.whats-your-sign.com/native-american-symbol-wind.html) it states that diamonds are the Native American's symbol for wind. Each point resembles the four winds (North, South, East, West). That tattoo also has a faint resemblence to the tattoos of the Shandians of Skypiea, who in turn are based of the Native Americans. Those in Skypiea use "dials" and have the ability to control wind currents (Breath/Jet Dials). Dragon's tattoo also looks very similar to the Native American symbol of "the medicine man's eye"