Doflamingo didn't really quit the Shichibukai- but even Sakazuki didn't know it?! In chapter 700, all of the Marines were talking about Doflamingo as if he had left the Shichibukai. At first it confused me a lot, but now I have a theory.
The Marines- for some reason, had only six Shichibukai: Mihawk, Kuma, Hancock, Buggy, Doflamingo and Law.
The Marines didn't want anyone to hear about Doflamingo not quitting the Shichibukai, so Brannew said that he did quit. And "that guy we talked about earlier" was Doflamingo, and he said that just because they wanted to be sure that everyone knew that Doflamingo's quitting was a fake.
That means the Marines really are working with Doflamingo. They want Caesar. But Doflamingo wants Caesar as well. And also Kaido.
I see the situation like this: Law get's beaten by Doffy. Fujitora takes Caesar. Doffy tries to steal Caesar from him but FUJITORA IS A BEAST AND HE KNEW THAT DOFLAMINGO WOULD TRY TO DO THAT SO HE WAS READY AND HE DEFEATED DOFFY. But not so easily. Doflamingo calls his customer, Kaido, and tells him that the Marines have taken the scientist. A huge clash takes place between the Marines, Kaido's army of SMILEs, the Dwarves and the Colloseum contestants.
Possibly, Jesus Burgess brings his whole crew and a battle between to Yonkou will happen. And in the middle of it- The Mera Mera no Mi and Caesar Clown.
Oh, and maybe the Kid-Hawkins-Apoo alliance will come join the mess as well!
Hawkins! >< }