Like i said, the difference could be that Scopper would be someone who maybe opposes this profecy thing and is doubtful that "the right one" will come.
Like how we know that something about the trip to Laugh Tale really upset Shanks.
There’s nothing in what we’ve seen of the Roger trip to Laugh Take to suggest that there’s any great plan for Gaban or that he’s any different to any other Roger Pirate in believing in what they read at Laugh Tale
For example Scopper was the one objecting to Oden joining them and leading the "anti Oden" camp until Oden earned his trust and friendship.
All Roger Pirates have approved of Luffy almost from the moment they met him, we have no opposite examples to that in the story.
It's basically not that much different from your proposal of making him someone who chose Blackbeard as "the one" instead of Luffy.
And for the purpose of seeing with his own eyes who makes it there and is supposed to fulfil this profecy, he'd have reason to settle in a place where he'd be sure to meet any "contenders" who have almost made it.
All Roger Pirates have approved of Luffy almost from the moment they met him, we have no opposite examples to that in the story.
It's basically not that much different from your proposal of making him someone who chose Blackbeard as "the one" instead of Luffy.
And for the purpose of seeing with his own eyes who makes it there and is supposed to fulfil this profecy, he'd have reason to settle in a place where he'd be sure to meet any "contenders" who have almost made it.
But this idea of Gaban being some sort of last presence at the end that will end up believing in Luffy anyway, it’s pointless. He’ll have beaten Kaido, proved himself to Shanks, proved himself to Rayleigh, gathered the Poneglyphs- why should Scopper Gaban suddenly feel himself so self-important as to be the last decider on who becomes Pirate King? Luffy will have the four Road Poneglyphs and the means to translate them, in the appointed time, if Gaban is standing in his way without being a real villain he’ll just look like an arrogant idiot.

