I am not sure if someone else has proposed idea, sorry if its already been proposed, lol.
But I was thinking about how Poseidon is a person (Shirahoshi), why couldn't Uranus be one as well? I believe Luffy might be that person; we already know Luffy has some unknown power that Gol D. Roger also had. And it is something similar to what the power of Poseidon is; the communication with sea kings. Though, something not synonymous with the Pirate King's power, is the ability to "read" Poneglyphs as well as write in that language, but that ability may not be realized as well.
Another thing I found interesting is that Pluto is a Roman name, while Poseidon and Uranus are both Greek names. So there is another connection between Poseidon and Uranus. Also Pluto is the god of the underworld, Poseidon the god of the sea, and Uranus the god of the sky/heavens.
With finding out that Uranus is the god of the heavens though, maybe we have yet to meet this person. And could it be someone from the sky islands?
What are your thoughts? :D