I learned not to disregard any bit of information Oda drops in One Piece. If the person who used to be 2nd commander was unimportant, Oda would said something like "ever since Charlie the 2nd commander died/retired, the spot has been empty". But this way, there's a lose end, and in One Piece, it means it's going to be tied, sooner or later.
This is the first time I thought Crocodile used to be one if Whitebeard Pirates. He's even standing on the side all in black, sort of like a black sheep compared to others.
But as fine and as interesting that sounds, there's a problem with how
Crocodile didn't seem to recognize Blackbeard in ID. And that's strange because Blackbeard has been part of Whitebeard Pirates for 20 years.
I guess there are slight possibilities that Crocodile left the crew more than 20 years ago or that he just never met Blackbeard but let's face it, those are astronomically small.
It could be Jewelry Bonney, but I kinda doubt it. Ability to make herself younger and the fact that she cried when WB died aren't enough.
Kaidou is a good possibility. We don't know when the WG branded 4 strongest pirates as emperors, but we do know that it's been less than 12 years ago. Why? Because that's when Shanks was at Luffy's island and I can't imagine him being able to go all the way to East Blue without offsetting the entire world balance and causing the Marines to chase him. Missing emperor would be noticed.
Since WB Pirates was probably the strongest pirate crew at it's prime, it's possible that Kaidou was a part of it for some time, but then went off to pursue his own ambition. I don't think WB would have stopped him or anyone who wanted to join him, as long as they didn't kill anyone. Still, I'm not that convinced of this, because it's just a shot-in-the-dark theory, with no real facts to support it. If it turns out to be true, I'll be really surprised.
Anyway, I voted "other" because I think it's very likely it's nobody from the list. It could be one of those shadow brokers, or even that guy Jack...