Eventually Luffy would become the pirate King but it's still too early for him or anyone in his crew to defeat a Yonkou. Most probably Big mom and Kaido are gonna negate each other
My take is that it has to be Zaraki who defeats Gerard or is the key to defeat him. The reason is that he is marked as one of the key dangerous elements by quicy leader (sorry forgot his name). And as far now they are the ones who have made the difference (though I guess Mayuri was not one of them).
I think Neco and the duke was referring to Roger when they said that there was another one who was able to hear the "loud voice", ayys it seems Roger's crew went to all the important islands Luffy had visited
There is another possibility. That person can be Rai's predecessor. It need not be true that Rai has been there from the inception of the nobles. So, its natural that there was someone else acting as the Noblesse.