Okay, time for my own F6 bounty predictions:
I tried to use the last number as an indication to in which order they became F6.
Who's Who - 781m - in between Cracker and Perospero sounds just fine for him, seems to be strongest of the F6 and a potential fight for Zoro
Sasaki - 672m - tried to put him in between Who's Who and Drake, due to his look it screams Jinbe fight
X Drake - 555m - fits his former bounty theme of 222m, plus over 500m seemed right for Mr. Diez Drake
Black Maria - 503m - still a bit over 500m
Ulti - 464m - seems like the newest member of the F6 along with her brother, but still shouldn't be far below Black Maria's bounty
Page One - 436m - if Sanji fights him again we might be seeing a Sanji bounty raise to 530m, which sounds about right.
In contrast to some opinions it makes no sense for the Straw Hats (excluding Luffy, maybe Zoro) to fight someone else. For example having Sanji face Jack or even Queen and beat him would elevate his bounty to 1b plus, likely to something bigger than Katakuri's (smells like bullshit at this point :D ) and that would result in the Straw Hats being presented as far to strong as they would have a claim to call them strongest pirate crew. In terms of story telling, that's idiotic.
:)