I know a lot of people think we may not have seen the real Big Bad of this arc. I agree.
Neither Decken nor Jones seem to be strong enough to put up a fight with Luffy post-timeskip. They look like Grandline-level threats, not New World-level, and I think even Caribou seems more dangerous than them.
With Jimbe now appearing, I wonder what kind of crisis could be there that he couldn't handle himself? Unless Jimbe is worfed by these newcomers (I'd hate to see that), I'm sure that this arc will have something a lot more dangerous showing up.
Maybe it's the source of Jones' drugs and money?
But… what if someone unsuspecting but already on the story becomes the main villain? Prince Fukaboshi looks like a good person, but maybe he could be wanting his fathers' throne and is using Jones for that (the "don't fight Hodi" could be a lie made by him)? He seems all too eager to get the Strawhats out of the island as soon as possible.
Or maybe Madame Shirley, after all she's the one who blamed Luffy, and it could be all a lie.
Or maybe both are plotting together. Both are "shark"-themed characters, and Fukaboshi was the first to act on Shirley's prediction.
(Not saying it's them for sure, just that it could be them. There's is, after all, a 'sinister force' working in Fishman Island… )