I just finished watching the raw of 290 (don't worry, I won't spoil anything this time ), and I noticed in the preview for the next episode looked just like the Detective Luffy special set in medieval Japan from a year or so ago. All of the Mugiwaras were back in the same clothes and everything, so it appears that we're getting a special sequel. Who'd a thunk?
And by the way, I have mixed feelings about this. The Detective Luffy episode was my favorite OP special of the four that we've seen, but I
really want to see the rest of the CP9 fights.
EDIT: I meant 290/291. I had just read chapter 438, so I got my hundreds digit wrong. If a mod can change the topic title, please do.