I thought about the whole crew fighting a Shichibukai. Except for the fact that: I don't think anybody but Luffy is going to beat the strongest of them, but he's already beaten Crocodile. If it took Luffy to beat the weakest, that suggests to me that they are all going to be beat individually by Luffy. Because if it didn't happen, than other crew would be get higher bounties than him. If Sanji beat Donflamingo, for instance, he'd worth over 300 million bounty. If Chopper beat Bear, he'd be worth 300 million. Is Chopper going to get a bounty 3 times higher than Luffy's? I don't think so.
The only exception to this is Mihawk who has a fight to settle with Zoro. Mihawk is going to be Zoro's final boss, ie, the man who he has to beat to become the strongest swordsman in the world.
Remember there are other ways to get a bounty.