It's usually not my style to revive a thread more than a month old, but since I'm a sucker for pirate lore, I feel that I should add a few pointers on who I think might be in league for future Shichibukai-dom. My vote goes out to Eustass Kidd, for a few reasons.
For starters, the pirates that inspired him were likely a Scotsman called William Kidd (for those of you with wikihunger, read more
here), and Eustace the Black Monk, a Boulognian aristocrate turned mercenary (
here). Like Eustass, William was appointed the nickname "Captain", owing to his ... enterprising nature, so to say. He eventually crossed the atlantic to the early Americas, where he privateered under the sanction of Richard Coote, the Earl of Bellemont and governor of a newly colonized and politically unstable New York. He engaged in several enterprises, including trafficing of exotic goods, and aiding in the construction of the Trinity Church that stands on the current day Wall Street, but what makes his case so special is that he was allegedly enlisted by the government to counter the spread of piracy around the remote reaches along the Cape and the eastern seas. He eventually went off the deep end when he started to attack British East India Trade Company ships, probably to sustain his expedition and his crew, and started to generally and indescriinately plunder and murder to stave off mutiny in the crew, which consisted largely of convicted criminals. That ruthlessness sound like anyone familiar?
Anyway, during his life of either willful or forced piracy he ended up accused of a great many murders, and the ensconcing of a substantial treasure somewhere in the Plymouth region. He was put on court of last resort back in the House of Lords in London, convicted of piracy, and subsequently hanged and strung up to dry along the Thames shipping lane as an example for other outlaws and aspiring pirates.
Eustace had a similar predilection for servitude to the British crown, and fought for king John of England in the battles of Normandy and Aquitaine in the early 13th century, and then switched sides to the French to rampage through the British ranks in the battle of Dover.
If Eustass's character is greatly inspired by these pirates (well, not in his looks, that much is certain), then we might actually see him as a Shichibukai one day.