Yeah, every arc tends to be unique and exciting in its own way. Oda had mentioned in an interview that he had figured out how the series will end like...around the time he started working on it. Since he's been having so much fun though, he decided to keep adding to the story in between.

The only thing that I could see getting harder to do as the series progresses though, is coming up with new nakama for Luffy. He pretty much has the entire roster that he's needed. The only unexpected one is Robin's role as an archaeologist...and he still doesn't have his musician yet. Increasing the crew and upgrading the ship helps a lot in keeping the story fresh, though, so I hope he keeps it up.

@Raven: Buggy's relationship with Shanks was revealed within the first 10 episodes of the series, I believe, when Luffy mentioned Shanks's name. It was there that they also explained how Buggy did not intend to obtain devil's fruit that he now possesses, and he blames Shanks for it.

@Ryougazell: Bartholomew Kuma. I seem to recall that another shichibukai was supposed to be a merman, too....