Rain water is still a possibility and its just a prediction dude.
Most of the fan base think that Dragon will either have a wind fruit, storm fruit, a weather fruit or he is the Uranus.
He IS Uranus? Interesting. I haven't heard that theory, but it seems plausible. My one issue is that that's basically what Shirohishi is with being Poseidon. It would make more sense to me if Uranus was a DF or something that GAVE Dragon the powers, as opposed to him simply being the ancient weapon.
Non Sequitur Idea Time:
Maybe it's his mullet that gives him power... I mean, mullets = hair metal, hair metal = AC/DC, AC/DC has a lightning bolt in the logo. Also, he has a face tattoo, which is kind of like KISS, so, according to the Monty Python school of logic, Dragon is the god of Rock 'n' Roll. And weather.
Also, the juxtaposition of the Roman Uranus with the Greek Poseidon bothers me, but that's just a side note.