It would certainly make him a more interesting character and Luffy would also have a reason to fight him beyond 'rivalry'. So your theory here would definitely be better story-wise.
But I dunno if I like it...
Shanks appeals to me because of his good guy trope, he has everything from character design to feats that make him one of the few forces in the world who isn't driven by just greed and lust for power (which includes most of the straw hats btw).
So what you're saying is better for the story, but I still hope it's not the case.