@IT'S:
Luffy promised to surpass the Red-Hair Pirates at some point. Doesn't mean they have to be the final villains. At this point, Blackbeard seems the strongest contender for Pirate King–next to Luffy, of course. They have conflicting ideologies. They have small crews.
Mihawk could duel Zoro while the rest fight Blackbeard's crew.
If blackbeard joins the shichibukai as he wants, he wouldn't leave his crew, or that's how it looks like. his crew are 5 and the shichibukai are 6 now, making a grand total of 11…
Wheren't 10 or 11 the crew members Luffy wanted?