User blog comment:Kaido King of the Beasts/Chapter 908: Imminent, Imbalanced, Improbable Whims/@comment-3185063-20180615081043/@comment-29325824-20180615081659

I've been wondering that as well. For a long time the honor of the big bad of the series seemed to go to either Sakazuki, Blackbeard, the Five Elders, or shared between them. But the revelation of Im clearly shows there are far greater things going on.

So, yes, I do think that Im is the likeliest candidate for main antagonist as of this point. Though I do wonder, does Im have the necesarry physical power to back it up? Considering almost nobody knows Im even exists, could he be even stronger than Kaido?