Disgarding the old man for a moment, I think the 7th shichibukai is SABO. It would all make sense. He probably is like an amazing haki user or something with no df abilities. But he will get a df ability, which guess which one? Yep that's ri ...

He did leave the navy. No one ever said why. Could be he was severely injured in a way where he can't use a bicycle anymore. If it is him, my money he shows up in a wheel chair. But why is he going to Punk Hazard?

Some of my opinions.
-They should just kill caribou, he's gunna leak the princess/weapon info.
-Aokiji should join up as new crew member.
-Big Mam is a friend via that one ugly chick from Thriller bark who gave straw hat her "mom ...