There's no way it's Vegapunk. They would just spend several pages in the earlier chapters outlining how Vegapunk and Caesar Clown don't get along only to should them teamed up. Also why would Caesar be after Franky's laser if he's working with Vegapunk. It's not him. Given all the information revealed about Vegapunk so far, I really doubt Oda would introduce him as an enemy.
I thought Aokiji from appearance, but Aokiji has never been sinister… so I doubt it.
So, the only reasonable candidates are:
Joker (There is literally no real information about this Joker character, so for all practical purposes it falls into the "someone new" category)
Kaido: I'd actually forgotten about this guy, but when someone mentioned him I looked him up, and other than a barely visible silhouette of his head there is no information about his appearance. Additionally one of his affiliates was a Cyborg... so maybe he's a sciencey-type character which could be a possible connection with Caesar.