It could be, I'd say the order of most likely is Vegapunk > Aokiji > Doflamingo > Joker.
Vega because there has been lotsa foreshadowing of him this arc, they are both "Docters".
Aokiji is related to the island, and has matching shoes.
Doflamingo has a connection to Law, but him being there doesn't make much sense, because Law isn't even suppose to be there(both shicibauki(or however the hell you spell it)) according to Smoker(iirc).
Joker, even though was mentioned, doesn't make much sense for him to be 'shadowed' because that just really isn't Oda's style for introducing new major characters. Plus him being there also doesn't make much sense. Unless he was scheming with the badguys.
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