@Razh:
The silhouette and the fact Vegapnuk is already a part of this saga in a way.
So all this baseless speculation comes down to these two points. Alright, let's tear this apart:
The shilouette has already been shown to be completely different. This isn't up for debate, we have eyes and can see that all the visible features as well as the size of the head are different. The only thing that's actually the same is the angle at which the shilouette is drawn. You can come up with any number of reasons why someone's features and head size would change, but those would only be your inventions and not based on anything from the manga. All we actually have at this point, is two (very vague) silhouettes that sort of look similar if you squint hard, but the actual details don't line up.
As for the second part… You know who else is also part of this saga in a way? Oden, his wife, his daughter, the two remaining samurai, every single Roger pirate, every single Whitebeard pirate, Gekko Moriah, Monet, Vergo, Caesar Clown, whoever the Rox are, and whatever void century characters end up being related to the "new dawn". Being "a part of this saga in a way" is not an exclusive list and does not narrow it down to Vegapunk in any way, shape, or form.
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@wolfwood:
Well if it has to be anyone other than Kawa-whatsit or Den-something i'd paint virtually anyone outside of the most important man in the world to be in a the man without a name in the iron mask style prison cell in the new world. Anyone with the time to spare to be forgotten in a prison camp, and as far as i know Enel has a pretty open schedule after attempting space suicide. I'd give him hundred to one odds
It could be the old captain of the Rox actually. That's someone who Kaido could conceivably want to hold onto for ages like that. Although my bet is still on the ponytail samurai.