User blog comment:Gibbz33/Who do you think is the 7th shichibukai?/@comment-5729687-20140214063438

The clue so far is that the 7th shichibukai is someone who cut off Zephyr's arm. So obviously this person is quite powerful.

I don't think it's shanks because he is already a yonko.

It will be a new character who most likely be around admiral level.