Ok simple enough. Die-hard proof Dr Hogback must be the next villain.
First off, he has a special and unique less. The scanlation I read use "fosfosfosfos" but it definatly stood out like Foxy's laugh did.
Foreshadowing of possible lies or deception. I think Usopp mentinoed it's strange he'd live here, have no knowledg eof the zombies afte rbeing away from the outside world for so long eventohugh his purpose is studying to bring people back to life like the zombies kinda were. THis points out some serious flaws in his expanation, and possibly could mean(since a zombie is his personal assistant anyways) that he is responsible for the hideous creations around the island. THey do have stitches, a classic Frankenstein feature.
Also, he didn't appear at first, was just mentinoed early on in the arc or something. Crocodile? Didn't see him until they reached Arabasta. Other villains also didn't enter until later in the arc.
He got a full-body "portrait" which only major characetrs get. First chapter has Shanks with a full image. That's jsut the first. Every major character at some point got a full-body image when they met the straw hats. Dr. Hogback got it on the page he was introduced, on the far right of the pag ein typical Oda style.
He's also very peculiar looking, not anything like a normal person or average pirate. He even seems to have given himself a new face.
Do I need any more of a reason?