I'm actually seriously playing around with the thought of Koala being a potential candidate. Here me out. It's been 15 years since the flashback. Who knows how she might have changed in terms of combative skill. Fisher Tiger expressed great admiration for her mentality. She might have turn out to be quite the radical fighter battling for the weak and oppressed be it by brawn or diplomacy. She might even re-enter the story once Oda shakes things up with new enemies, new challenges, new trouble. It would definitely be a more original way for Oda to introduce a potential candidate. Random flashback character becoming someone important to the story.
But this is all based on speculation and a bunch of ifs. I don't really care for it. Just an interesting idea. Jimbei is way more likely at this point.
@RobbyBevard:
Plus its pretty obvious there was a big split when Jimbei became a shichibukai. What exactly caused the split and the terms we'll find out soon, but I'd imagine it was conflict between him and Arlong, but it might have also been part of the shichi deal.
Or maybe because the crew were disgusted with him for choosing to work for the humans.