Regarding the last spoilers about the Rain God Zaza, we have learned that it's the biggest fear or nightmare of the Tenryuubito. That may be the reason, why they live on the top of the Red Line above the clouds, where it never rains. And when they're going to the lower realm for example Sabaody, then they are wearing these bubbles around their head, especially Killingham. The other holy knights and five elders do not, because maybe they are not scared about rain. For example Gunko said she fears Nika the most.
Furthermore Monkey D. Dragon is the biggest enemy of the world government and most wanted person. There it makes sense that Dragon got a power, they are fearing of. On his first appearance in Logue Town suddenly it was rainy and stormy. Later in Chapter 587 we saw the ship of the revolutionary army and it seemed pitch black, like the description of Laffite when he was talking about the Man marked by flames.
Actually we don't know anything about Dragons Power or whether he is marked by flames, but all the clues suggest that he must be that person and also owns the last Road Poneglyph. And honestly I don't think that we will get a very new character introduced at this point. Just like Luffy is the reincarnation of the Sun God Nika, Dragon could be the reincarnation of the Rain God Zaza or he got the power of the Hito-Hito-No-Mi Model Zaza.
What's your opinion about this theory?